Generally I'll see it as I do it. I touch type so unless I'm typing from copy, which I very rarely have to do these days (pretty much never) I'll see it and backspace to fix it.
If I'm typing from copy I can usually still tell when I've cocked it up so I'll either just backspace and fix it, or if I'm not sure I'll look at the screen.
I'm not even close to as good at typing as I was when I was doing layout for a living though. I was pretty rad then.