The phrase "dead giveaway" means something that is very obvious or easily noticeable, revealing the truth or giving away a secret or hidden fact. Full definition
«I'm a female» = dead giveaway that you are, in fact, a man trying to pose as a woman. (omgchronicles.vickilarson.com)
There's no secret handshake, but pockets full of pods, some crazy chile t - shirt, or a Peppers of the World book worn - out by heavy studying are dead giveaways of those fiery fans. (fieryfoodscentral.com)
The Michigan Manufacturer license plates on the Toyota Highlander and the camouflaged sports car are dead giveaways for what's hiding under the camo. (autoweek.com)