How to tell if I've eaten gluten

Thanks to all of you for your help with the wheatgrass question. Now, here is another. What type of reaction can I expect if I eat something with gluten? I try to be really careful, but I'm not sure if accidents have happened or if it is just normal healing. Also, I have been having an off again / on again pain in my upper right hand side of my stomach, under the top of the ribcage. I'm wonder if this is normal healing, a hint that I have had gluten, something unrelated, or something else..
Also, I have seen some instant mashed potatos, no where do they list any gluten containing product. Under allergens, it just lists milk. Yet doesn't say gluten free, so I have passed. Anyone have any hints on this?

Thanks again... Tweet