1)Best and easiest method is
getconf LONG_BIT
2)Use uname command for the same
uname -m
x86_64
Tells this is a 64 bit system
3)uname -ar
Linux localhost.localdomain 2.6.32-220.el6.i686 #1 SMP Wed Nov 9 08:02:18 EST 2011 i686 i686 i386 GNU/Linux
Tells this is a 32 bit system