How to know if I have an allergic reaction to alcohol?

Couple things. Couple things here-first of all, there are medications well know to interact with alcohol: 'disulfiram reactions'. Common one: Flagyl. I saw this on an airplane; little old lady was beet red after a mixed drink. No harm. Also some people develop nasal congestion - wicked stuffed up - trick is certain drinks bother them, and not others. Or simply exaggerated flushing, such as with Rosacea. See MD.
Alcohol Intolerance . Symptoms of Alcohol intolerance may include: headache, flushing, hives, nasal congestion, GI upset, nausea/vomiting & low blood pressure. You could be allergic to alcohol or it's ingredients such as yeast, grains, sulfites, histamines & other chemicals.