@tamsin Another option (because testing food is expensive) are 'gluten immunogenic peptides' or GIP test kits.
These can only test retrospectively, but allow you to look back at what you have eaten / where you have been and then infer likely sources of food contamination.
(I found it was a game of cat-and-mouse for the first couple of years, and A LOT of filtering out well-intentioned but uninformed misinformation)