I've kinda been wondering this too. I don't have many headaches or blurry vision. It's just that I can't read some things on the white boards at school. Maybe because of the marker, maybe the handwriting, maybe the distance, maybe the position or view, or maybe I just need glasses. I don't know! What do you guys think?
If what you see is blurry but you can see well closer to you, you are probably nearsighted. If things up close are blurry but you things at a distance are clear, you are probably farsighted. There are other conditions, including glaucoma which is extremely serious but gradually dims your sight. Best thing to do is get a full eye examination and then ask how often you should do that.