Visibility Test for Units not Players

Does anyone know a way to test if a unit is visible to another unit? I tried making a dummy player that just gets vision of one unit at a time, but visibility doesn't change instantly, so I cant just turn it on and off in my trigger.

I have a system of light sources that players are granted vision of if the player is in its line of sight (accually another unit sits by it that has a longer range than the light, cause you can see light sources outside of thier light emmition) The players have no line of sight otherwize. It works really well with one unit, but once vision is given, it cant be taken away quick enough to test the next light source.

I'm thinking there might be a way for the ward to effect every unit visible in some way, but i dont think it there is a way to do that.