How to tell between head cold and allergies?

I thought I was just getting seasonal allergies, with sneezing and watery eyes. But then it went into my head and now I’m stuffed up and have a cough. Today, I started getting kind of sick to my stomach. I tried Nyquil last night. Didn’t help and I’ve taken some Zyrtec today but still feel cruddy.

Anyway to tell the difference between a cold and allergies?