When you see the little blue flare fly straight up, it's a UAV.
If it is an enemy UAV, you typically can't target it (although on occasion I've been able to, not sure why), allied UAV's will have the little blue triangle over them!
How to hit UAV's:
Aim above the little blue flames, the jets on the UAV are not part of it, and do not take damage when shot.
A small dot will appear in the center of your cross-hair.
The range-finder is also an indicator you're on target.
Happy Hunting!
Edited by The Flying Gecko, 26 October 2014 - 09:46 AM.