How can you tell the difference between viral pink eye, bacterial pink eye, and allergies?

Some differences. Generally viral pink eye typically produces redness, watery eyes and doesn't need treatment except for herpes. Bacterial pink eye causes redness & purulent discharge (puss), often "matting the lids/lashes together" in the AM or after a long nap -- needs antibiotic drops. Allergies almost always involve both eyes with redness, some watering, & can be quite itchy. Consult doc if you have it.