The best solution would of course be implementing pain sounds ~ ala Doom ~ so that hits can be identified.
There are several orthogonal solutions:
1. hit points flying out of the damaged unit, like in final fantasy games.
2. info window, exposing all the dice rolls the game engine makes.
3. hit animation. I.e. displace damaged unit for a second.
4. sound. (having just them will offend the deaf users)
5. blood and gibs.