Dick, Philip K. . "Impostor." Astounding June 1953. Coll. Robots, Androids, and Mechanical Oddities, and The Best of Philip K. Dick. Rpt. Souls in Metal, q.v. under Anthologies and Collections.
The protagonist is accused of being an imposter: not a human but an alien robot carrying a U-bomb. The protagonist's recognition of his true nature occurs just before he blows up.