A University of Washington study concludes that people have an inbuilt 'gaydar' (an abbreviation of the slang term 'gay radar') in which they are unconsciously able to determine whether or not someone is a homosexual from merely looking at their photo for one second.In the study, 96 black-and-white photos of gay and straight men and women were shown to 129 students, who were then asked to guess their subject's sexuality.
In two-thirds of cases, they were able to tell if a woman was gay or straight, and they accurately judged 57 per cent of the males they saw.
Even turning the photos upside down did not stop ability of viewers to correctly guess the subject's sexuality.
In May 2010, Dutch researchers concluded that "Gaydar," the supposed innate ability of gay people to identify other gays even in a crowd, may actually exist.
More perceptive and detail-oriented people may be more likely to pick up on subtle clues in other people that they may be homosexual, the study found, which helps them search out gay friends and sexual partners. (read the study: Link) published by the Public Library of Science.
Here is a 2007 video in which gay's are asked about gaydar. Pretty amusing!