Well, you could take him in to a doctor for a penile plethysmograph test. The accuracy of those tests is open for debate, but some supporters of the technology believe that measuring subtle changes in penis while subjecting the patient to a set of images and sounds presents good data. You definitely will get data, that is for sure. Of course, the test isn't all the subtle and is highly intrusive. If your son is straight, he'll probably hate you quite some time. If you son is gay, he'll probably hate you for quite some time. But at least you'll have a data set with a printout and everything.
Outside that, what the other suggestions here are aren't really "tests", they are "traps."
What William Said.