You can do a skin test on her arm with a dairy product to check for skin reactions.
When it coems to eating challenges, I would test dairy with a whole food product such as just cheese or milk instead of in a mixed food. That is what we were instructed to do by our Naturopath. Then you wait a few days to look for any signs (and check out her diaper too, for any effects!) I would continue to test it gradually by offering one serving, waiting 2-3 days. Then 2 servings of a dairy food in the same day, and waiting another 2-3 days.
Btw, yogurt and cheese tend to be easier to digest than milk, you might want to start with those.