Hello, I'm sorry for the delay in responding, thankyou for the tag Jennifer
I have been diagnosed with Coeliac disease since I was 14 years old. I was mostly diagnosed through blood work which was off the charts (on 6 seperate occasions over the course of a year) . I did have an upper scope but this was inconclusive and as i was so young at the time my consultant didn't want to keep putting me through unpleasant procedures without good reason. I responded very well to the gluten free diet and my blood tests improved rapidly. The blood tests are improving in accuracy all the time and sometimes biopsies can come back negative even though coeliac is present. My peadi GI explained that sometimes, the inflammation is too far down to reach or the inflammation can be patchy which can mean they sample the wrong area (sometimes inflammation can only be seen on a microscope). Coeliac disease can also occasionally go into spontaneous remission, so you could have it, but still get negative results because its "gone to sleep ". This doesnt mean it's gone for good though, and patients should still strictly stick to the diet as it can re-activate at any point. It is also important for patients to remember that coeliac can sometimes be silently active which would mean you don't get outward symptoms (pain, nausea, D. etc) but internal damage is still present. I've spoken to many people who stopped their diet because they "didn't feel unwell" only to land themselves with severe malnutrition and osteoparosis because of the amount of internal damage gluten has caused.
If you get a positive blood test, my advice would be to have it repeated in a month or so, and see if its still positive, then possibly trial gluten free, have another blood test and then see if your TTG levels have improved. Don't change your diet until you have had the tests completed, as changing it too soon will disrupt the results.
If you do get diagnosed then yes, Jennifer is absolutely correct, double check everything, I've seen a lot of packaging that says a product is "suitible for coeliacs" but it actually contains vinegar or wheat, so read the ingredients. Also, in a resturant, trust your instincts, I've had bad experiences where I've been told something is safe when its not and almost landed in the hospital, so ask questions, look at how clean things are, see how the staff handle being asked specific things and always trust your gut.
For reference, I was diagnosed with Crohn's when I was 19.