It is more about the symptoms then tests that are ran. My daughter had it when she was younger. Painful when laying down is one, they have a feeling of acid building in the throat and almost feel like it is burning. Sometimes even puking up stomach acid. It would be bright yellow. Upset stomach at night well after eating and worsening when laying down position. It isn't anything they can diagnose other then symptoms being given. If it is super severe they will do an endoscopy to see the lining of the stomach and make sure the acid hasn't done damage to the lining or esophogus. You can try giving a tums too to see if that relieves symptoms, a sure indicator it is acid related.
My daughter was on liquied kids Zantac for several months and actually grew out of it thankfully. She was not eating solids so it was more a matter of what I ate as she and getting from the breast milk. I have it and what I am doing that helps is Probiotics, like Activia yogurt every morning or even capsules that you can sprinkle in foods, yogurts and not eating greasy or fried foods of any kind. Try drinkable yogurts for kids too.
Chocolate is another thing that can stir mine up and I love it so much I just take a acid reducer well before I am going to have a piece, hee hee.