Uname

uname is a command to display information about the running kernel. You can use it to find out if your system is 32bit or 64 bit.

Example:

tweedleburg:~/ # uname
Linux
tweedleburg:~/ # uname -a
Linux tweedleburg 3.4.11-2.16-desktop #1 SMP PREEMPT Wed Sep 26 17:05:00 UTC 2012 (259fc87) x86_64 x86_64 x86_64 GNU/Linux
tweedleburg:~/ # uname -p
x86_64
tweedleburg:~/ # uname -m
x86_64
tweedleburg:~/ # uname -i
x86_64