Food diary is what my allergist recommends. Some reactions, like migraines, don't appear for 2 days, and the only way to look back into your history is to write it down.
After you get a list of meals or foods you can do challenge testing (see the search box for information). But only try this if you don't have severe reactions. An example is say you reacted to a hamburger. You then have to break it down into the bun, the meat, the condiments and seasonings.
These tests worked wonders for me. I found a single ingredient (sulfite) causes most of my reactions. Eliminating it has been literally a lifesaver. Before I knew about it I ate a bowl of cereal with dried fruit and my stomach and esophagus triggered my coronary arteries to spasm (via the vagus nerve). This led to a heart attack (pinched artery) which I was lucky to survive. Look up cardioesophageal reflex.