IDIOMS

dead giveaway

(idiomatic) Something that discloses, usually unintentionally, a fact or an intention.

Example:   That disguise is a dead giveaway! Choose something less obvious.
2008, New York Times, March 5
  "Instead of slowing down his arm when he throws it — a dead giveaway to hitters — Igawa had better arm action, Eiland said"