I don't know anybody who hasn't wanted to test something we know bothers us, and I can't advocate it for anyone but myself.  However self-testing ("challenges"  in allergy terminology) can be a quick way to test things.

Just be sure to do it in a safe way.  If they can be absorbed through the skin, then I'd try making a paste of it and covering it with a band-aid or plastic wrap for 24-48 hours.  If you want to try it orally, you'd want to start with only a tiny amount of it.  You can put it between your lip and teeth and leave it there a while.  Local tingling, itching and/or swelling would give you an answer right away.  If you get any systemic reaction, I wouldn't try that substance again until I'd conferred with my doctor.  Here are some things to do to ensure safety, mostly common sense:

If a person tends to have hair trigger for anaphylaxis, it might be wise to do challenges only under a doctor's supervision.

Use only a pure form of the medicine or food that you think is bothering you.  Otherwise, it's impossible to know if it's an additive, preservative, or the main ingredient causing the problem.  If a food is used, it's hard to know for sure whether it was other things in that food causing the problem or specifically sals.

Keep medicines nearby for at least a couple of hours after trying the substance, including antihistamines (H1 and H2) and Epipens.  If there's potential for a delayed reaction, carry them with everywhere (yes, even the bathroom) for 24 hours.

Have someone else present for at least several hours after trying something new.  Delayed reactions are not uncommon.  Don't be alone overnight.

If living far from an Emergency Department in a hospital, try to go somewhere close to it for the experiment.

A person might not need that extreme level of caution, but it's better than being surprised by a big miserable reaction!

Some aspirin, especially liquid, are pure salicylates without added ingredients.  Always check the inert ingredients before taking any new medication.