In regards to hidden barley there is a product called Rice Dream milk which uses a barley enzyme and doesn't mention it on the containers. You wouldn't know it because it says Gluten Free on it, and the ingredient involving use of the barley in it is partially milled brown rice. The company says they extract the barley and test but barley testing can supposedly be very inaccurate. I suggest you never use this product as I have heard of quite a few people (including myself) having reactions to it.
Best to read everything on all packages. Quite a few times I checked all the ingredients while in the store then got home and later saw some little small writing on it that says processed on equipment which also processes wheat.