How can I tell which of two very similar linux kernels I booted from?

Solution 1:

You can "tag" your kernel to identify it later on.
In menuconfig, go to "General Setup" -> "Local version - append to kernel release"
Any string you enter there will be displayed by uname as suffix to your kernel version.

Solution 2:

$ ls /proc/fs

...should show cifs if you're on the new kernel.

Solution 3:

Another answer to this is cat /proc/cmdline

This shows the path to the kernel and any command line parameters that were used.

example.

cat /proc/cmdline

BOOT_IMAGE=/boot/vmlinuz-3.2.0-17-generic root=UUID=b33290c0-553a-4fd1-af00-b82017923b88 ro ipv6.disable=1 splash quiet vt.handoff=7