Android - How to tell current frequency band?

Based on avirk's hint, I found a way. First press the code *#0011#

GSM example:

GSM900: IDLE
T: 10, B: 10
Rx Pwr: -94, Rx ual: -
Rx Lev: 21, Tx Lev: 0-
Speech VER: AMR EFR FR
VOC: AdaptiveRate
TS: 0
Temp: 68 Batt: 71 LNA: 0
Service: Available

It is obvious that it's using 900MHz band, from the "GSM900".

UMTS WCDMA example:

WCDMA: Idle
Rx CH: 10612, R: -94
Tx CH: 9662, Tx Pwr: -
EdIo: -6, RSCP: -98
SpeechVER: AMR EFR FR
RF: Sleep2
L1: PCH_Sleep
Drx cycle: 64, PSC: 392
Temp: 67 Batt: 69 LNA: 0
Service: Available

UMTS can use two different physical layers (low level radio) UMTS-FDD and UMTS-TDD. Android just shows WCDMA, which I think is the same as UMTS-FDD, but perhaps it can refer to both of them. By looking up the channel number here you can tell the frequency bands and whether it is UMTS-FDD or UMTS-TDD.

In the above example it is using UMTS-FDD, the down-link channel (Rx) is in the 2100MHz band and the up-link channel (Tx) is in the 1900MHz band.

You can try dialing

*#*#4636#*#*

and then going to "Phone Information".

That will show details about the network like cell tower ID and network type. As you can see from the below screenshot, it shows "EDGE" mode :-

Also, this may only work on stock Android builds.

LTE Discovery app tells this for 4G/LTE.