Before I was diagnosed, I started seeing a naturopathic doctor (after 20+ years of allergies, including 5+ years of allergy shots). Before making any changes, she did a blood test to see if I had any food sensitivities. Gluten and wheat were off the charts, so we did a Celiac panel - came back positive. In addition to giving up gluten, I also had to give up dairy, yeast, citrus, and oats. I did this for 2 months, and then started to challenge the different foods. Next month will be my 2 year mark.
I have been able to add back some dairy - goat cheese and hard cheese only. No milk/butter/soft cheese - they all cause my allergies to be much worse. I can go from feeling fine to having severe post nasal drip with just a few days of having a little bit of cheese on a salad. In the past few months, I have cheated a little on the milk/butter front - mostly when I am out and don't want to deal with it. I should try to get back on the wagon though
The other thing that sometimes causes problems if I have too much of it is yeast. I was eating a lot of kombucha and kefir (coconut milk variety), and my sinuses started acting up. So I gave it up for another few months, and did another challenge and it was OK.
I am still on all my allergy meds (though I am hoping to get off them someday), but now they are at least under control. It used to be that I would be on all of the meds, and still have more symptoms that most people.
If you can't get the blood test, you could always try an elimination diet for a couple of months, and start adding foods back one at a time. I think I usually did a challenge for 3-5 days - except for dairy. Dairy i did for 5-7 days and did it incrementallly (hard cheese, then soft cheese, etc).