I may have a corneal abrasion. How can I tell?

Hurts. This is a loss of the protective skin covering the cornea. The cornea is well endowed with pain fibers and an abrasion will hurt significantly. If the history suggests an abrasion (i just scraped my eye; the edge of a paper hit my eye; a tree branch swung into my eye, etc.), and the eye hurts an abrasion is likely. See an er doctor or an ophthalmologist for treatment.