Knowing can be more suspicious than going to a dressing room or bathroom that is surrounded with mirror. Usually, the first question asked here is as to whether there is a two way mirror used in the said space. Good thing, it is possible to know this. It would only take a matter of checking of the mirror is actually transparent. The way it is observed should be learned this way. There are various techniques to learn this. The same is also true with the wall behind it. For instance, the fingernail test is no longer new to some. This may be an accurate way of learning if the mirror is actually two-way or not.