I heard that you can tell if the scanners are even on if you use bluetooth headphones. When you walk through them, the music sounds weird/quieter. This happens when I go through scanners on my campus' library (which are always on), so I feel this could help you know if they're even on at all since I've heard some stores don't even turn them on. I've also heard most won't even go off if it's something under a certain price. I don't know if any of that is true, but maybe look into that more.