The only clear way to be sure who is the enemy, is to check the markings. Using zoom in-game helps in this. Learn to identify the different markings in QMB.
The next best thing is to identify the plane type. If you're flying a BF-109 and see a Spitt...
This is most often reliable enough way to identify the enemy. So learn to id the different types too.
Ofcourse there is servers where there are mixed/unrealistic plane sets where you cannot definitely say what planes the are the enemy (unless you check the enemy airfields and what they've got).
-LLv34_Untamo