Knowing the current target of a unit?!

What i wanna do is to check if a unit's target is in a unit group. Sounds easy doesnt it? Yea i thought so too.... Untill i tried. WEU does have advanced triggers for checking this, but they were obsolete/disabled. So now i tried to add the units to an unit group array as soon as they were GIVEN The target unit (there are only 5 different places to attack, so only 5 targetrs). But then, of course, i encountered a problem with that too. They are randomly given one of these 5 units as target, and therefor i cannot know which target they were given, EXCEPT with the trigger i wanted.

So any ideas?