As far as I know, that's the only way to do it. There are tests available from naturopaths etc but I understand most of these are not evidence based or have actually been discredited by research. I went off dairy for 4 or 5 days with no benefit and concluded that it wasn't the culprit. Then I was told that it takes quite a few weeks before it's completely out of Mum's system and bub's gut has a chance to heal. So I gave it another shot and sure enough, a few weeks later I started to see real improvement. I still wasn't entirely convinced - maybe he was just maturing and getting better on his own. Then one night I was out to dinner and decided that I would indulge in a dairy-heavy meal as my test. 48 hours later, my poor bub was beside himself. No more cheese for me until he weaned at 18 months!