id [USERNAME] name will show the users id (uid), the users main group id (gid) and other groups id (groups).
So for me:
[craig@tuxx craig]$ id craig
uid=5008(craig) gid=5008(craig) groups=5008(craig),500(shared),5003(cg_admin),5004(usag)
Shows that I have an id of 5008, my main group id is 5008, and my other groups have an id of 500,5003 and 5004.