If you aren't sure just call it how you see it, more often than not people get you.
For example on valhalla, you can call left trees and people will know where you mean in-game. Calling something is better than nothing at all, and calling early is even better! Don't wait until you're dead if you see them 10 secs before then call 'my X'. Sometimes it's un-avoidable though.
Try to call everything out even if you know your team-mates or yourself can't shoot the guy. At least your team builds a picture of where the opposition are on the map.
The most important thing when calling out is to remember that you don't "have" to kill the guy, just call-out and back away or wait for a team-mate.
