I have no idea how they test for it. I have it and have had it as long as I remember. I did find an online deal where they tested to see what frequency the sound is. Apparently different people hear different sounds. Anyway, they played the most common frequencies and, if it hit the right one, it kind of cancelled out the tinnitus. It was kind of amazing. I don't know that I'd ever heard "nothing" before!
Maybe the test is something like that? What ever it is, good luck with it and please report back what you learn!