 What you do is you shine your phone's flashlight through them. If one side of the mirror is more brightly lit, you have a problem. Next, this is really important. Check the devices connected to the rental's wireless network for anything strange. Now, if all of this is just totally freaking you out, you can pick up a hidden camera detector on Amazon. They're not expensive. They're about 50 bucks. And if you're looking for my recommendation on them, just head over to commando.com and in the search box, just type in a hidden camera detector.