If you're just interested in whether the currently running system is 32 or 64 bit, then 'uname -m' will tell you either i686 or x86_64 depending on whether you have 32 or 64 bit cpu.
Unfortunately if you want to know if the system *could* be 64 bit, you need to check the cpu flags. The flag 'rm' means its 16 bit, 'tm' means 32 bit, and 'lm' is 64 bit.
`grep flags /proc/cpuinfo`
rm: real mode, 16 bit
tm: transparent mode, 32 bit
lm: long mode, 64 bit