There are a few good ideas I have heard here...do her ears turn like other cats' do, even if she doesn't look that direction?

Also, if you can get her alone, and behind her, shake a can of pennies or rocks...see if she reacts. Or clap your hands. Actually, do that with a few of the cats there, as you cannot isolate her. They will react - and she---?

Or shake a granola bar at her and see if she RUNS LIKE HELL!

Disclaimer: That last suggestion has not been verified by independent medical research.