There are evil skin packs you know. I'm sorry but I don't have the link anymore (virus got my comp), but there's one that turns the evil people red, and one that inverts them. I'd get the inverting one because the red one is missing some character (I think Piccolo).
Or, for an even cheaper solution:
-Everyone on the server agrees to certain classes
-Pick a unique class (ie: Be the only Trunks) and say so to your team, so at least you won't get hit
-If team damage is off, pay close attention to what happens when you hit someone. If they bleed, nail that person until they're dead. If they don't get hurt, you know it's a friend, but keep watching that person carefully. If someone else shoots your friend, observe what happens to them. If they bleed, kill the SOB who shot them. If they're fine, you have another friend to keep track of and get info. from.