People who travel a lot have been posting tips and tricks to help keep you safe and here's another one. A flight attendant who travels a lot posted the routine she goes through every time she checks into a new hotel and it included checking to see if the mirror is a two-way mirror. She says she uses the 'fingernail method' which I had to look up.
Apparently you press your finger against the glass and if you see a gap between your finger and the mirror, you're fine but if your finger appears to be directly against the glass, it could be a two-way mirror. Some sites say this might be unreliable, so here's some other ways to tell. Stay safe friends!