I have puffy eyelids. How can I tell whether I am allergic to my mascara?

Stop the mascara. Stop the mascara and see if the puffiness goes away!
Try different brand. If a person thinks mascara is causing eyelid allergies, she can stop the mascara a few days to wait for her eyelids to normalize. Then she can try a different brand of mascara. If she wants to "test" herself with the old mascara, just wait until the eyelids are normal. Then resume the old mascara on only one eye's lashes. If that eyelid reacts, but the other lid is fine, then an allergy is likely.