foods? do an elimination diet
other things, well just experience. you get stung by a wasp and your throat closes up then that tells you something right?
also allergies are very tricky in that you might be allergic that very minute and the next day the same allergen won't effect you. this is why allergy tests at doctors are a SCAM. they only tell you what you are allergic to AT THAT VERY MINUTE the test is given. which is useless