How do I find the boot parameters used by the running kernel?

You can run cat /proc/cmdline.

Example:

[01:31] ~ $ cat /proc/cmdline
BOOT_IMAGE=/boot/vmlinuz-2.6.38-7-generic root=UUID=025c4231-b7bb-48bf-93e9-d20c5b5ce123 ro crashkernel=384M-2G:64M,2G-:128M quiet splash bootchart=disable acpi_enforce_resources=lax vga=792 vt.handoff=7

An alternative way is to check the output of dmesg (line 5 here):

$ dmesg | grep "Command line"
[    0.000000] Command line: BOOT_IMAGE=/boot/vmlinuz-3.19.0-33-generic root=UUID=81dba11f-f76e-4ed4-8120-e6da6328b1ee ro

But note that this might not work if many things have been logged (e.g. if the machine has been running for a long time) because initial startup lines may have been pushed out of the ringbuffer.