i like the civ idea, there should be AI civilians, and real people, the real people, have things like radios so they can report things such as troop positions, if someone kills an 'enemy' civilian, they get 2 points, else all the civs pull out guns and start shooting the dude..
The obvious way to spot bad guys would be with bombs and such, if they have a large bomb strapped to em, then shoot em.. if you see them talking into a radio (which they should try to conceal) shoot em too.. give em all pistols too, just in case
obviously you wont know if a civ is on your team or not.. you just gotta be careful, if you get a radio message from someone who is giving radio messages, then.. DONt SHOOT HIM! :p