Here is a typical crowd of heterosexuals. There is one fly in the ointment...in other words, there is one gay person in the picture. It should be fairly simple for anyone who has gaydar to pick out the culprit:

Here is a typical crowd of homosexuals:

Note that they are dressed completely differently from the heterosexuals and they they also appear to be exhibiting very different behavior. They also seem to be wearing their hearts on their sleeves, which is why they are holding their arms out like that, so that their hearts do not fall off and make a mess on the ground.