In OXCE, you can press Alt and a small arrow will point at the alien.
In addition to this, OXCE (maybe OXC too, I don't bother with that build) also shows revealed enemies in green (the vanilla behaviour of flashing red boxes is retained if the currently active soldier can
see the enemy), bleeding allies in blue, and allies with high Stun damage in purple. (Unfortunately, mind-controlled aliens count as allies for this last I checked)