How do I tell what is causing my allergic reaction rash?

Timing. It depends on the timing. The contact with the allergens, either through skin or oral, usually happens days or weeks ago. Person's recall history is crucial. Tests are unreliable or not available.
See a Doctor. You need to see either an allergist or a dermatologist. Figuring out a rash can only happen when someone lays eyes and hands on it.