What is the process for diagnosing a food allergy or intolerance?

[hazelnutlatte]

I am beginning to suspect that my dd (7 months) has some sort of food allergy or intolerance. She has been in a constant cycle of diarrhoea (15 or more poos a day) followed by constipation for the past month, and also has a severe nappy rash. I have been to my GP a few times, they just tell me it's a stomach bug and to keep her hydrated, and have prescribed cream for the nappy rash.
I'm wondering If these symptoms could be due to a food intolerance, but don't want to start cutting things out of her diet because dd is quite small and eats very little, it is also a challenge to get her to drink much milk (she is formula fed and the milk refusing has been an ongoing problem since she was about 2 months old)
I don't want to try and diagnose my dd myself, I would like the GP to be involved, or to refer dd to the right person. Can anyone explain to me what the process is (or should be) in this sort of situation?

I had my GP finally referred DD to a Gastro Paed at our local hospital to get tested for Coeliac. all he had to do was write to them explaining why he was referringus. they did a blood test. It was a long hard slog though, 12 months to convince them to refer us, 6 weeks wait for appointment, then another 2 for the blood test, then another 6 for the results.

The blood test checked for major allergies as well as coeliac.

Keeping a food diary is helpful too, it helps identify possible triggers, and the hospital may ask you to start one anyway,

forewarned is forearmed as they say.