How to know a Unix/Linux system is a 32/64 bit architecture?

To know whether given Unix/Linux machine is of type 32/64 bit architecure, use the command

getconf LONG_BIT

It outputs correspondingly 32 or 64 bit.

For AIX machine use "bootinfo -K" command for same purpose

Reference: http://www.unixguide.net/unixguide.shtml