How can I tell whether I am allergic to garlic?

See allergist. See your allergist. He/she can run an allergy test on you to see if you're allergic. But if you develop a rash, sinus congestion, watery eyes, or anaphylaxis whenever handling or consuming garlic, that's a big hint.
Garlic Allergy. Eliminate it completely from your diet for 4 weeks, then add it back. See if your symptoms go away (off) and return (on) when you restart. You can also see an allergist for testing.