Well good news, there's the simple way, and the hard way for sorting you out.
You can get a food intolerance or food sensitivity blood test done. These will take a small blood sample and test you against 60 - 200+ foods, colours, preservatives etc (the more you pay, the more they'll test ;).
option 2: an Elimination diet
1. Keep a food dairy
2. Make a note of any discomfort or symptoms you feel; from bloating to lethargy
3. Identify all possible culprits
4. Eliminate all culprits until you no longer suffer any symptoms
5. Go TWO full weeks without any suspect foods
6. One at a time, slowly reintroduce each of the suspects and check for a reaction. Reintroduce no more than one every couple of days for accuracy.
7. If you don't suffer any reaction, you may reintroduce that food into your diet. If not, eliminate the food for a couple of months before trying to reintroduce it again. The hope is you can restore your gut to a point where you can hopefully consume a wider selection of the culprits once your digestive health improves.
8. If you are reacting to multiple food sensitivites or intolerances, I'd strongly suggest seeking a Naturopath for ongoing help, this can be a long process and good guidance and adherence is paramount.
Some reactions may even run in to the next, meaning you may be intolerant to two foods but only blaming one... As mentioned, it can get tricky but not impossible!
What gets measured, gets managed
You must quantify what your eating so patterns occur and assumptions made. Start by eliminating the common culprits and keep notes of anything suspect, and of course seek further medical advice if needed.
Most Common Causes & Symptoms of Food Intolerances
What is a Food Intolerance