Check health of eMMC

You probably want to use the "mmc" utility from mmc-utils (https://packages.debian.org/stretch/mmc-utils). I have, however, not yet seen a write and/or bad block counter.

# mmc extcsd read /dev/mmcblk1
=============================================
  Extended CSD rev 1.7 (MMC 5.0)
=============================================

Card Supported Command sets [S_CMD_SET: 0x01]
HPI Features [HPI_FEATURE: 0x01]: implementation based on CMD13
Background operations support [BKOPS_SUPPORT: 0x01]
Max Packet Read Cmd [MAX_PACKED_READS: 0x3f]
Max Packet Write Cmd [MAX_PACKED_WRITES: 0x3f]
Data TAG support [DATA_TAG_SUPPORT: 0x01]
Data TAG Unit Size [TAG_UNIT_SIZE: 0x03]
Tag Resources Size [TAG_RES_SIZE: 0x03]
Context Management Capabilities [CONTEXT_CAPABILITIES: 0x05]
Large Unit Size [LARGE_UNIT_SIZE_M1: 0x00]
Extended partition attribute support [EXT_SUPPORT: 0x03]
Generic CMD6 Timer [GENERIC_CMD6_TIME: 0x19]
Power off notification [POWER_OFF_LONG_TIME: 0x19]
Cache Size [CACHE_SIZE] is 4096 KiB
Background operations status [BKOPS_STATUS: 0x00]
1st Initialisation Time after programmed sector [INI_TIMEOUT_AP: 0x50]
Power class for 52MHz, DDR at 3.6V [PWR_CL_DDR_52_360: 0x00]
Power class for 52MHz, DDR at 1.95V [PWR_CL_DDR_52_195: 0xdd]
Power class for 200MHz at 3.6V [PWR_CL_200_360: 0xdd]
Power class for 200MHz, at 1.95V [PWR_CL_200_195: 0x00]
Minimum Performance for 8bit at 52MHz in DDR mode:
 [MIN_PERF_DDR_W_8_52: 0x00]
 [MIN_PERF_DDR_R_8_52: 0x00]
TRIM Multiplier [TRIM_MULT: 0x03]
Secure Feature support [SEC_FEATURE_SUPPORT: 0x55]
Boot Information [BOOT_INFO: 0x07]
 Device supports alternative boot method
 Device supports dual data rate during boot
 Device supports high speed timing during boot
Boot partition size [BOOT_SIZE_MULTI: 0x20]
Access size [ACC_SIZE: 0x08]
High-capacity erase unit size [HC_ERASE_GRP_SIZE: 0x01]
 i.e. 512 KiB
High-capacity erase timeout [ERASE_TIMEOUT_MULT: 0x03]
Reliable write sector count [REL_WR_SEC_C: 0x01]
High-capacity W protect group size [HC_WP_GRP_SIZE: 0x10]
 i.e. 8192 KiB
Sleep current (VCC) [S_C_VCC: 0x08]
Sleep current (VCCQ) [S_C_VCCQ: 0x07]
Sleep/awake timeout [S_A_TIMEOUT: 0x12]
Sector Count [SEC_COUNT: 0x00e90000]
 Device is block-addressed
Minimum Write Performance for 8bit:
 [MIN_PERF_W_8_52: 0x0a]
 [MIN_PERF_R_8_52: 0x0a]
 [MIN_PERF_W_8_26_4_52: 0x0a]
 [MIN_PERF_R_8_26_4_52: 0x0a]
Minimum Write Performance for 4bit:
 [MIN_PERF_W_4_26: 0x0a]
 [MIN_PERF_R_4_26: 0x0a]
Power classes registers:
 [PWR_CL_26_360: 0x00]
 [PWR_CL_52_360: 0x00]
 [PWR_CL_26_195: 0xdd]
 [PWR_CL_52_195: 0xdd]
Partition switching timing [PARTITION_SWITCH_TIME: 0x03]
Out-of-interrupt busy timing [OUT_OF_INTERRUPT_TIME: 0x05]
I/O Driver Strength [DRIVER_STRENGTH: 0x1f]
Card Type [CARD_TYPE: 0x57]
 HS200 Single Data Rate eMMC @200MHz 1.8VI/O
 HS Dual Data Rate eMMC @52MHz 1.8V or 3VI/O
 HS eMMC @52MHz - at rated device voltage(s)
 HS eMMC @26MHz - at rated device voltage(s)
CSD structure version [CSD_STRUCTURE: 0x02]
Command set [CMD_SET: 0x00]
Command set revision [CMD_SET_REV: 0x00]
Power class [POWER_CLASS: 0x0d]
High-speed interface timing [HS_TIMING: 0x02]
Erased memory content [ERASED_MEM_CONT: 0x00]
Boot configuration bytes [PARTITION_CONFIG: 0x00]
 Not boot enable
 No access to boot partition
Boot config protection [BOOT_CONFIG_PROT: 0x00]
Boot bus Conditions [BOOT_BUS_CONDITIONS: 0x00]
High-density erase group definition [ERASE_GROUP_DEF: 0x01]
Boot write protection status registers [BOOT_WP_STATUS]: 0x00
Boot Area Write protection [BOOT_WP]: 0x00
 Power ro locking: possible
 Permanent ro locking: possible
 ro lock status: not locked
User area write protection register [USER_WP]: 0x00
FW configuration [FW_CONFIG]: 0x00
RPMB Size [RPMB_SIZE_MULT]: 0x20
Write reliability setting register [WR_REL_SET]: 0x1f
 user area: the device protects existing data if a power failure occurs during a write operation
 partition 1: the device protects existing data if a power failure occurs during a write operation
 partition 2: the device protects existing data if a power failure occurs during a write operation
 partition 3: the device protects existing data if a power failure occurs during a write operation
 partition 4: the device protects existing data if a power failure occurs during a write operation
Write reliability parameter register [WR_REL_PARAM]: 0x05
 Device supports writing EXT_CSD_WR_REL_SET
 Device supports the enhanced def. of reliable write
Enable background operations handshake [BKOPS_EN]: 0x00
H/W reset function [RST_N_FUNCTION]: 0x00
HPI management [HPI_MGMT]: 0x01
Partitioning Support [PARTITIONING_SUPPORT]: 0x07
 Device support partitioning feature
 Device can have enhanced tech.
Max Enhanced Area Size [MAX_ENH_SIZE_MULT]: 0x000124
 i.e. 2392064 KiB
Partitions attribute [PARTITIONS_ATTRIBUTE]: 0x00
Partitioning Setting [PARTITION_SETTING_COMPLETED]: 0x00
 Device partition setting NOT complete
General Purpose Partition Size
 [GP_SIZE_MULT_4]: 0x000000
 [GP_SIZE_MULT_3]: 0x000000
 [GP_SIZE_MULT_2]: 0x000000
 [GP_SIZE_MULT_1]: 0x000000
Enhanced User Data Area Size [ENH_SIZE_MULT]: 0x000000
 i.e. 0 KiB
Enhanced User Data Start Address [ENH_START_ADDR]: 0x000000
 i.e. 0 bytes offset
Bad Block Management mode [SEC_BAD_BLK_MGMNT]: 0x00
Periodic Wake-up [PERIODIC_WAKEUP]: 0x00
Program CID/CSD in DDR mode support [PROGRAM_CID_CSD_DDR_SUPPORT]: 0x01
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[127]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[126]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[125]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[124]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[123]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[122]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[121]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[120]]: 0x02
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[119]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[118]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[117]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[116]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[115]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[114]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[113]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[112]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[111]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[110]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[109]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[108]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[107]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[106]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[105]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[104]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[103]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[102]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[101]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[100]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[99]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[98]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[97]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[96]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[95]]: 0x02
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[94]]: 0x01
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[93]]: 0x04
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[92]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[91]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[90]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[89]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[88]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[87]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[86]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[85]]: 0x03
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[84]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[83]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[82]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[81]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[80]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[79]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[78]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[77]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[76]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[75]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[74]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[73]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[72]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[71]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[70]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[69]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[68]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[67]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[66]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[65]]: 0x00
Vendor Specific Fields [VENDOR_SPECIFIC_FIELD[64]]: 0x00
Native sector size [NATIVE_SECTOR_SIZE]: 0x00
Sector size emulation [USE_NATIVE_SECTOR]: 0x00
Sector size [DATA_SECTOR_SIZE]: 0x00
1st initialization after disabling sector size emulation [INI_TIMEOUT_EMU]: 0x0a
Class 6 commands control [CLASS_6_CTRL]: 0x00
Number of addressed group to be Released[DYNCAP_NEEDED]: 0x00
Exception events control [EXCEPTION_EVENTS_CTRL]: 0x0000
Exception events status[EXCEPTION_EVENTS_STATUS]: 0x0000
Extended Partitions Attribute [EXT_PARTITIONS_ATTRIBUTE]: 0x0000
Context configuration [CONTEXT_CONF[51]]: 0x00
Context configuration [CONTEXT_CONF[50]]: 0x00
Context configuration [CONTEXT_CONF[49]]: 0x00
Context configuration [CONTEXT_CONF[48]]: 0x00
Context configuration [CONTEXT_CONF[47]]: 0x00
Context configuration [CONTEXT_CONF[46]]: 0x00
Context configuration [CONTEXT_CONF[45]]: 0x00
Context configuration [CONTEXT_CONF[44]]: 0x00
Context configuration [CONTEXT_CONF[43]]: 0x00
Context configuration [CONTEXT_CONF[42]]: 0x00
Context configuration [CONTEXT_CONF[41]]: 0x00
Context configuration [CONTEXT_CONF[40]]: 0x00
Context configuration [CONTEXT_CONF[39]]: 0x00
Context configuration [CONTEXT_CONF[38]]: 0x00
Context configuration [CONTEXT_CONF[37]]: 0x00
Packed command status [PACKED_COMMAND_STATUS]: 0x00
Packed command failure index [PACKED_FAILURE_INDEX]: 0x00
Power Off Notification [POWER_OFF_NOTIFICATION]: 0x01
Control to turn the Cache ON/OFF [CACHE_CTRL]: 0x01
eMMC Firmware Version: 002122  AS

Above answer is right that mmc-utils is the way to go. The version in git (https://git.kernel.org/pub/scm/linux/kernel/git/cjb/mmc-utils.git/) reports DEVICE_LIFE_TIME_EST_TYP_{A,B}, where 1 is 0-10% reserved blocks used, 2 is 10-20%, etc. I presume the flash storage in question has to support this field before it can be reported.