a dead giveaway
A clue, detail, action, etc. that obviously reveals something else. Your shoes are totally sticking out of the blankets! That's a dead giveaway that you're planning on sneaking out as soon as mom says good night to us. The homemade card was a dead giveaway that they had forgotten to buy us a gift.
dead giveaway
something that reveals a fact or an intention completely. The car in the driveway was a dead giveaway that someone was at home.
giveaway
n. something that reveals a fact that was meant to be concealed. (Often with dead.) The way he was walking was a giveaway to the fact that he was the one who was injured.