I've heard that motels are sometimes used as meth labs; are there any signs that I could look for to know if my room was used to cook meth?

Look for stains on the carpet. Also stains in your sink or bathtub, is there any rusting, look for left over packages of the ephedrine, cold medications. If you own the hotel, and or establishment there are a list of items you should pay attention to.
Meth lab indicators.