@tamsin I'm not sure if you can get testing kits.
Our eldest is very sensitive to gluten, so we stick to things that are clearly gluten-free (fruit, veg...) anything with the crossed grain logo and we avoided anything containing oats.

Having separate spreads, toaster bags helps. (Apologies if you're already doing this, but these are things that caught us out!)

Oats contain avenin, a protein similar to gluten, that most coeliacs are unaffected by. We suspect our eldest was possibly sensitive to it as well.

Any Q's, just ask.

https://www.coeliac.org.uk/
#coeliac