detect two way mirror

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Is it really possible to detect a two way mirror? The one that seems to be ordinary but someone can actually see you on the other end. A circulating Facebook post says that all you need to do is place the tip of your finger and if there is a gap, it is a genuine one. However if there isn't, the post says it may be a two way mirror (I think one way mirroris the proper term but for consistency, I'll use the term on that post) Another is the miniature camera in the fitting room. That same post told that if you can't make a call, chances are, there is a hidden camera installed. Do you think these are true or just another Internet memes? Have you read these posts?