My children and I are gluten/casein free. easy child appeared to have problems with gluten since she was a baby. I had her tested for celiac disease several times over the years but it was negative. Then I found a lab on-line which does a different kind of test and that I could order myself. When I did that, she was positive for gluten intolerance.
Since she was positive, it is recommended to test the other family members. When I tested difficult child and myself, we were also gluten intolerant. When I went gluten free, I noticed within 2 days that I needed to get off my Lexapro because I felt "giddy". We noticed pretty quickly with difficult child that it made a difference.
This lab also tests for casein intolerance and I had tested difficult child for that at the same time. I read on another forum that casein made the posters feel "mean" and I thought that might apply to difficult child. She was positive but we didn't make her give that up at first. When it appeared that the gluten free diet wasn't totally eliminating difficult child's symptoms, we tried eliminating casein (milk protein) too. Many people with gluten intolerance are also casein intolerant.
With my easy child, her stomach hurts if she eats gluten, casein, or soy so I went by that but did test her to confirm.
Having the positive lab results gave me the motivation to try eliminating gluten for myself and difficult child. But it was actually seeing the results that made me believe. The lab is not accepted by most mainstream doctors because the methodology hasn't been peer-reviewed yet. I think their testing shows that 50% of the population is gluten intolerant. So I think anyone could just try doing the diet without getting the testing and see if it worked.
You do have be extremely strict because even a trace of the allergen can cause symptoms. I once had some french fries fried in the same oil as something with wheat and I was depressed and irritable for 2 1/2 weeks. It's an awfully hard diet to do if it doesn't show results but for us, the results have been worth it. The only time difficult child acts like she used to is when she has any milk or gluten.
If you are going to just try different things, I would start with gluten, then do milk, soy, and the other top 8 allergens. Other people have noticed problems from food coloring and corn syrup so you could try those, too. I would give it at least 2 weeks of no mistakes and then re-introduce and see what happens. We have never deliberately re-introduced because we make enough mistakes that I know what happens and I don't ever want to bring that on on purpose.