To determine what version of the Linux kernel you are currently using, you can use the uname command as per the examples below.
$ uname -a
This will output the host name of the computer, Kernel version, and the CPU architecture.
$ uname -a Linux localhost.localdomain 2.6.32-279.19.1.el6.x86_64 #1 SMP Wed Dec 19 07:05:20 UTC 2012 x86_64 x86_64 x86_64 GNU/Linux $
localhost.localdomain being the host name of the computer
2.6.32 being the kernel version (the rest relates specifically to distribution based kernel customizations)
x86_64 refers to it being a kernel supporting 64bit CPU architecture.