Strontium-90 is a beta emitter, apparently, so you could just measure Alpha, Beta, and Gamma and detect false positives due to elevated background radiation that way. If you're in the air you're probably much closer to your Beta source as well so you could detect it above background due to distance. Gamma is essentially light so it falls off with the square of distance. But Alpha and Beta are probably much worse than Gamma in that respect, so I'm guessing this is fine most of the time and then they rule out false positives by looking at the total background in each type of radiation.