Question: If I get hit in the eye, how do I know when it's an emergency?
Answer: If you get hit in the eye, the first thing you should do is go look at your eye in the mirror. If where the eye's normally white you see blood, or if your eye is so swollen shut that you actually can't see the eyeball -- those are two reasons why you should actually go to the emergency room. If your vision's blurry in the injured eye, or if you have a lot of pain when you move that eye around, those are other reasons that you should get your eye checked out. If you actually have a black eye, it's probably okay, but it could be a sign that there are broken bones underneath. So you should probably get that checked out as well.
Whatever you do, do not put pressure directly on the eye if you're going to put a patch or bandage over it because that can actually make the injury worse.