You might wonder how I know to avoid all the foods I avoid. By keeping a food diary.
I learned about food allergies in my teens. I had a severe reaction to clams causing my throat and tongue to swell. The doctor suggested I avoid all shell fish, since most people who are allergic to one type of shell fish are allergic to other types as well. This is a cross reactivity food allergy. I later discovered that while I am allergic to mollusks (clams, oysters, scallops, etc.), I could eat lobster and shrimp.
I was fortunate to see a doctor who was thorough and asked about all of my symptoms, not just the ones that landed me in urgent care. The doctor explained that I was allergic to clams (an immune system reaction), but that the other symptoms we discussed were not likely allergic reactions. A food allergy is an abnormal response to food triggered by your immune system. A food intolerance is NOT triggered by your immune system.
A battery of tests revealed that I had a few allergies, but not nearly enough to explain the symptoms I frequently had. These included hives, itching in my ears and throat, headaches, blisters in my mouth, gas, intestinal cramps, blurred vision, asthma and chest pains. The doctor asked me to keep a detailed diary of everything I ate, came into contact with and my symptoms. I recorded dates, times and amounts.
I returned to the doctor after a month. We went over the diary and discovered I had several food intolerances. Over the years I have used the food diary to help me identify all the foods that give me problems.