Celiac or a bug?

How do you know when a kid is really sick?
Earlier this week Bridget (non-celiac) was complaining all day about her stomach hurting. We thought maybe she's a little constipated so we gave her some Miralax in apple juice. I guess it helped. She has stopped complaining about her stomach anyway.
The next day Kate (celiac) complained her stomach was hurting too. She was at grandma and grandpas for a while and slip-ups can happen when there are a lot of wheat-products around. She has diarrhea and bloating when she has a lot of gluten, but it's hard to tell if she is really sensitive yet. She didn't have diarrhea.
Anyway, Kate is the kind that asks for medicine because she likes the taste. She may have been glutened or she may have had a bug or she may have needed some extra attention. I don't want her to use celiac as an excuse just because she can. But I also don't want to not believe her when she is really sick. How can you tell if your kid is really sick or just had a little gluten?