How can i tell if it's a corneal abrasion or conjunctivitis?

You can't : Corneal abrasions and conjunctivitis may both present with a red eye. It is important to see your eyecare provider right away as the condition may be serious.

Pain: The cornea has more nerve endings than any part of the body, even your fingertiip. Corneal abrasions are painful, may have reflex tearing and light sensitivity if located in the central cornea, vision can be affected. Abrasions can become infected, resulting in scarring and vision loss. Conjunctivitis is associated with a red eye and there can be some discharge. It is usuallly self-limiting.

Pain: Abrasions are typically from trauma, foreign bodies, or even recurrent erosions with typically one eye involved, can be very painful, the vision can be significantly affected, there is a lot of clear watery discharge. Conjunctivitis is typically bilateral, vision is usually good, and the symptoms progress. Usually the white of the eye is red/pink (i.e. Pink eye).

Both need exam: Conjunctivitis normally has a discharge with possibly a mild foreign body sensation. A corneal abrasion has a significant foreign body sensation with tearing , and occasionally photophobia.