Bring doesn't detect them.
Command doesn't detect them.

Is it possible to detect them?

I do not believe they're detectable as hallucinations, but detectable as the units they represent. What are you trying to use this for, anyway?

Actions work on them, but conditions don't. The best way to detect if there is a hallucination is to center a location on the hallucinated unit type and have a real one in the rightmost of the location. If none of that unit exists in the location your are centering then you know it is on a hallucination.
This is only the best solution is certain circumstances where the area is very controlled. More information would help get a better solution.

To expand on this, if your real unit is to the left of the halu and you can't or don't want to use the give action, you can always try using mobile grids (assuming this is for Hero Sanctuary) to center some location in which you will look for halus.

Your solutions seem to always involve mobile grids

Mobile grids seem to be handy too ;p

Thanks for the attempts to help everyone, but i figured out my own way to deal with this. This can be locked now.

p.s. I AM > TRIGGERS!!!! (hS is awesome!)