@kkgirl can you get copies of your old lab results? You can get them from your doctor (and you should keep copies). Do your parents have them? Can you check to see if you actually had the entire celiac panel? I ask because I test positive to only ONE on the panel (DGP IGA), yet I had a intestinal damage. Did you have biopsies six years ago or just a screening celiac blood test?
As you say, there's no test, it's diagnosed by exclusion, so there's no metrics to check to assess compliance other than anecdotal response to symptoms. Conversely, I've not seen anything that says that NCGI can resolved or be cured either. This paper gives a good summation.
Basically, there's not much research period! That is changing though, I think there's research underway which could help give answers.
First, do you have celiac disease? Have you recently had a celiac antibodies follow-up blood test panel to see if gluten has been actually getting into your diet by accident or through cross contamination? If you haven't had the test, you should ask your doctor for one. You could also also ask for a follow-up endoscopy to see if you intestines have truly healed. This will rule out if gluten is really the problem.
If you don't have celiac, I assume you have Non-celiac gluten Intolerance because no one would stick to the gluten-free diet for six years unless you have had reductions in symptoms. There is no test for NCGI unfortunately. So, it is hard to say if gluten has been sneaking into your diet. You really are going to have to try to figure out if you are getting gluten into your diet.
If gluten is not the problem, then your doctor needs to check you for other issues, like SIBO.
There is a test to check for SIBO. Has this been done?
Once a celiac, always a celiac. There is no cure for celiac disease except to remain on a gluten free diet for life. I am not sure about NCGI. Not much research has been done. Maybe others can chime in?