it would be on the bumper. i'll give you an example: my HMMWV was M10, and i was in the 632nd Maint Co. on the left side of the bumper were the parent units and on the right was the unit and number, kinda like this:
___________________________________________________________
3 DSB 87 CSB (-------------------space--------------------) 632 MC M10
___________________________________________________________
of course, every installation has different standards, but from what i've seen, they're all pretty close. there's always some sort of unit designation (3 ID, 1 CAV, 3-3 AD, etc) on the bumper number so you know who's unit that vehicle belonged to.
that's why i say a lack of unit designation may mean it last belonged to a civilian or dod contractor. usually i only see a number (or nothing) on the bumper on dod vehicles, but sometimes it'd say DOL or DRMO or something. just a thought.