How can you tell if you scratched you cornea or if you simply have something stuck in your eye?

My left eye has been in serious pain since I woke up. I spent at least the last half hour probing around my eye to see if I could get anything out of it. Looking in a magnifying mirror, I could not see anything.
Then I started to wonder if I had scratched my cornea somehow. Is there a visible mark when that happens?
Anyone have any other ideas as to what’s wrong with my eye?
(It hurts the most when I blink, if that helps).