The supposed ability to detect whether or not a person is queer by observing that person.
1993 October 6, Peggy Boucher Murphy, “Re: Paula Poundstone?”, in soc.motss[1] (Usenet):
maybe she's bi? she sure tingles my queerdar in a big way.
2004, Rick Hoffman, account of the March for Women's Lives, Memorandum from the Pro-Life Alliance of Gays and Lesbians, Number 46, Summer 2004, page 3:
During that march, my queerdar was on full throttle. I saw HRC equality and rainbow stickers all over the place.
2009, Jason Edwards, Eve Kosofsky Sedgwick, Routledge, →ISBN, page 46:
Are there such things as 'gaydar' or 'queerdar'? That is to say, do some individuals possess an uncanny ability to recognise someone or something as gay or queer and to acknowledge it publicly, […]