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Here are all the facts, experiences, techniques and media/links we have on:

Gaydar

With contributions by black moon. - Thanks!

An internal sensor that allows you to know if someone is homosexual. 

thoughts

Of course it's more a myth than a fact because most homosexual individuals aren't that simple to recognize. There is no typical "homosexual" way to behave.

You can see if someone looks at you "too long" as usual for heterosexual individuals but this isn't a certain method either because someone might have homosexual feelings without define him-/herself as "homosexual" or you might misinterpret his or her looks.

The only certain way to be sure that someone is homosexual seems to be to watch him or her doing homosexual activities, ask him or her - which might be difficult - or to approach him or her in a explicit way - which might be dangerous, or make someone feel uncomfortable.