The best way to find out if a food disagrees with you is to keep a diary, then gradually eliminate the culprits, if problems persist try another food and so on. Trial and error not magic.
I'm sure some of the allergies they say I have came up via blood tests initially (eg horse - they are very right with that one!), from bloods they took at my pre-op, testing for latex allergy etc... They then did the skin prick ones with lots of different allergens.
sorry, I hit enter too soon. Seriously if you think you are allergic to one thing then just cut it completely out of your diet and see what happens. It can take as long as 2 months to have any effect. If you feel that you are allergic to something and don't now what, then you stick to a diet of things that you KNOW you are not allergic to. If you aren't sure then you eat a really plain diet like rice and water and one source of protein (honest)