How do you know to go to the hospital after you've been hit in the head?

Natasha Richardson's skiing accident scared the hell out of me. It just made me think, what if that was me? I mean, sure, I don't know how hard she fell, but I've been hit in the head a few times, one of which was by a soccer ball, how would I have known to go to the doctor after being hit in the head? It's just a really eerie situation that makes me wonder.