There is no reliable medical test to give evidence of a food intolerance yet there are millions who declare with incredible anecdotal evidence that they suffer from intolerance to foods. An intolerance to lactose in milk is a bit different in that the cause is known. An insufficient amount of the enzyme lactase is proven to cause lactose intolerance. The mechanism of action for every food intolerance is not known. Those with food allergies can be tested by various medical tests to prove an allergy.
The wonderful thing about our bodies is that they tell us most of the time when there is something wrong. Even though we may not be able to eloquently articulate to our medical care providers that there is something wrong, we still know. We live in our bodies and feel every cramp, experience every problematic bathroom moment, feel the constant fatigue, endure the headaches, and suffer with skin that is dry, cracked, flaky and irritated. We know that there is a cause to these things.
After years of eating and digesting we begin to establish that there is a pattern. We don’t usually keep a log of details, but we recognize patterns. Maybe we notice that every single food that has any part of an egg in it will make us sick to one degree or another. Once there is a conscious suspicion then resolution should be sought.
Begin resolution by abstinence from all instances of ingesting the suspected offending food for a few weeks. Pick only one suspected offender (the most likely first). If improvement is demonstrably measurable, then you have your answer.