Unless it's a straightforward and clear allergy but you suspect a more complex intolerence picture, I would second the nutritionist idea and a general exclusion diet. There are several methods of testing, which people have their opinions on but as the prick test can only test what you "ask" it to and you can have false positives and negatives, it can be worth looking elsewhere.
Sometimes the body is stressed and can be helped by removing the difficult to cope with foods (wheat, yeast, dairy, egg, orange, chocolate, tomatoes, sugar) and rotating foods that are a challenge to cope (this is very individual and can include other grains, other deadly nightshade family - potatoes, aubergines, peppers - mushrooms, soya, etc).
A nutritionist can also advise on suppliments including long chain fatty acids, vitamin c, probiotics and enzymes that can support the process. Once the body has a chance to heal, intolerence to foods is likely to decrease. Some tests allow you to reassess tolerence levels on a regular basis so that you do not introduce foods until the body is ready to cope with them allowing the body to fully heal before putting it under stress again.
Practitioners operate in different ways but the results / strategies are ofter similar. Finding someone you trust is important, get a recommendation if you can and ask about their experience and qualifications. It's important to have a good relationship with them so chat to them first so you can feel comfortable with asking questions and accepting their advise.