Is it more likely to have an allergy to caffeine, or stimulants in general? Skin rashes appear on lower face and upper chest. Testing for this?

Quite rare. I don' recall ever seeing someone allergic to caffeine. As each stimualnt drug is quite different chemically from others an allergy to all stimulants is even less likely. There is also no skin or blood test for such an allergy but that doesn't mean it can't be done. A double-blind challenge is the method used to establish a relationship between a cause & an effect. Your local allergist can help.