Who? (The Gregg Press science fiction series)

Martino was a very important scientist, working on something called the K-88. But the K-88 exploded in his face, and he was dragged across the Soviet border. There he stayed for months. When they finally gave him back, the Soviets had given him a metal arm...and an expressionless metal skull. So how could Allied Security be sure he actually was Martino ?