ummm... when everything starts going blurry. But it helps to do eye checks here and there, coz you never know. I have glasses ( And i am NOT a nerd) and I'm short sighted. Everything just goes blurry that's how you tell.
The unsigned answerer
It's not. You don't get bad eyesight from reading - you get it because of genetics!
my eyesight is very bad
Looking just in front of them is blurry, even with big eyes they have bad eyesight
yes it did
By reading in the dark.
no but bad for your eyesight. that what happened to my friend instead she was on her i touch all day she has negative eyesight for both eyes
He has bad eyesight.
It means that your eyesight is bad
Yes
yes it's genitic
Yes.
Things like bats and moles.