An unmistakable and definitive clue: His blushing was a dead giveaway (1882+)
As with phone bills, major expenses or bills for jewelry and other gifts are a dead giveaway.
Of course, the loud buzzing of such drones and towels being blown away would have been a dead giveaway to the pool girls.
He drew near the rocket, thinking that the open hatch would be a dead giveaway.
Despite the tight beam, any communication could be a dead giveaway.
Radar pulses emanating from a presumably lifeless planetoid would be a dead giveaway.
The accidental use of a first name could mean a dead giveaway at the wrong time.
Their bubbles would rise right past the frogmen, a dead giveaway!
It was a dead giveaway when Young struck both cartels simultaneously, if Hunter had read the data correctly.
The red sweater with the terrycloth border and the white pants with a shortened left leg were a dead giveaway.