You'd need to devise some sort of experiment. One way would be to record your % proc chance without mastery (over a very long time), then add mastery and see if it changes significantly. I'm sure someone can come up with a better way, but this is just what I thought of.
Yea the only other way I can think of is to read through some combat logs and see. Though you'd have to go through a ton to have a chance of proving that it doesn't (you'd only have to find one occurrence of it happening to prove it does) and it would be a pain in the butt to do =/