actually in the case of COD3, i have had no trouble at all figuring out who is who. it is quite simple. i usually go by 3 things
1. just go by who is across from you and who is shooting in your direction.
2. this is kinda of hard, but sometimes you can hear which direction the germans are yelling from.
3. i always go by the way your target goes red if your aimed at an enemy. and if you aim at a friend the target goes green and their name comes up next to it.
im not sure if its the same with yours becuase you might be able to turn that off i guess. but its so easy to tell who is who. the only time ive killed my own teamate was during a really eradic battle and everyone was moving and i just started shooting all over with my machine gun lol