http://www.esquire.com/entertainment/movies/a53558/a-major-die-hard-plot-hole-was-just-explained-29-years-later/
Recall thinking Mcclane must have ESP to be sure.
I always took it he kinda knew from the accent.
I thought it was because everybody in the theater was yelling, "He's the Bad-Guy"!
I always figured he thought he recognized the voice despite the accent (because he had been hearing it all night on the radio), so gave him an unarmed pistol to test him.