90% of food allergies are due to gluten and dairy products. It is well worth everyone cutting these out of your diet for a week. Failing that, keep a diary of when you consume them and notice if any problems consistently occur in the next 6 hours.
Not all allergies have obvious effects. I have a long-running back problem which improved tenfold when I stopped eating gluten.
Nor are allergies necessarily permanent. I managed to cure my own gluten allergy using timeline therapy (crazy huh?). There is also an NLP Allergy Cure which works somewhat like the Fast Phobia Cure.
Minor allergies can become much worse if our immune system is weakened.
Allergy testing can be quite expensive and I'm not sure if anyone has tested its validity. Donna Eden's book, Energy Medicine, describes energy testing, where you would eat, or simply smell various foods, and after each, lift a heavy weight with your non-dominant arm at a 45 degree angle to the direction your facing. The more easily this happens, the worse a food is for you.