Symptoms of a food allergy vary widely and can look like the symptoms of other medical conditions, so a diagnosis from a medical professional is important.
An elimination diet is one way to test for a food allergy. In an elimination diet suspect foods are eliminated from the diet for two or three weeks, with symptoms monitored before and during the trial.
If the symptoms improve it's likely one of the foods eliminated is the culprit. This can then be tested by reintroducing foods one at a time.
The most definitive tests for food allergy are ones that identify whether there are increased IgE antibodies (created by a true allergy) in the blood, as a result of the food.
Skin prick tests are useful for diagnosing soy allergy. In this test a drop of a potential allergen is placed on lightly scratched skin. If IgE antibodies are raised the skin will become irritated.
If you are diagnosed with a soy allergy, you'll need to eliminate all soy from your diet. Soy is widely used in food processing, so you'll need to start reading food labels.