1)My doctor said one of the best ways to find out is to take it from your diet for a month. Be really careful about it during that time. Then after a month have a "party" day. On that day you eat as much of that food that you possibly can. Totally pig out on it. See if your symptoms lift, and if they come back on the party day.
For example, I had suspected a dairy allergy/intolerance for my son. He had a chronic cough, asthma, colic and severe acid reflux. The cough wouldn't go away no matter what we did. Antibiotics didn't help. Nothing. So one day, we took him off of dairy completely. If it had whey or casein or even anything that hinted of dairy I would not feed it to him for a month. After a few days the cough was gone for the first time in months. I continued for a month. Then one day I gave him cheese, milk, you name it. Anything that had dairy in it, I fed him like crazy. Within an hour or so the cough was back. He had severe diarrhea. Within a few hours he had a rash. When I took the dairy back out, the cough, diarrhea and rash went away.
Every great while I let him have something with milk in it like pumpkin pie, hoping he will grow out of it. So far the symptoms still come rushing back when we try it. He has been on a limited diet for many years now. It hasn't been fun taking such a yummy food group away from him. However, the improvement in his health has been remarkable.
Keep in mind that if you have problems with more than one food and don't know it. . . this method may not totally work. Let's say you have a problem with dairy, wheat and eggs. If you only take out dairy, you might not see a dramatic improvement because the other foods are still causing you problems. I have a friend who tried this with a variety of foods. In the end one of the big culprits was citrus. This was something totally off the radar. So although this can be helpful at times, sometimes it is still not very conclusive. If you suspect multiple foods, it may be a good idea to take them all out. However,
2.If you are really unsure of what to try, I'd suggest the elimination diet here. She is a nutritionist and this may be just the help you need to try it.