It's not true, as guest pointed out, it merely distinguishes between a first or second surface mirror. In the case of a "two way mirror" (more properly, a "one way mirror", or even more properly, a "half silvered mirror") it will appear to be a mirror from whichever side is illuminated more brightly, and the other side will be visible from whichever side is more dim. Thus, if you put your face near the mirror and cup your hands around your eyes to block out the light from the side you are on, you can see the other side, even from the more brightly illuminated side.
This distinguishes between a first surface and second surface mirror, not necessarily a two-way mirror. Unless you've compared both types, you probably won't be able to tell the difference with this test. Actually, kind of useless information, really.
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