MP games are usually divided between East (Russian) and West (NATO) teams. You should already be familiar with the differences in uniforms from playing OFP's SP missions to be able to identify friend or foe. If the soldier you see is distant and you can't make out which side he/she is on, well, life is full of important decisions, isn't it?
If your problem is that you've gotten used to playing OFP in sissy - er - cadet mode and you've become dependent on those nice target boxes that display when you place your cursor over an enemy on the screen, well, you've got to get used to the fact that most MP hosts prefer playing in veteran mode, where these are non-existent. Of course, if you yourself are hosting, you can host in wimpy - er - cadet mode and the target boxes will display for all the lamers - er team players.