For example, if you see that someone is standing in one spot for a long time and you then scope with your sniper from across the map and they run away right when you start to aim, this is a sign of a screen watcher.
Another example would be a Matt Cowan. This is when you know your opponent does not have radar and you did not shoot your weapon, so your opponent has no way of knowing where you are. A screen watching opponent would take a peek at your screen and then throw a "random" frag grenade across the map and easily kill you. This is another sign of a screen watcher.
Tyler: Wow, I didn't screen watch at all. Actually, I went into a random building and saw you there so I knifed you.
Stru: *singing* SCREEEEEEN WAAAATCH!
Paulo: Wow, Stru, how did you know I was there?
Stru: I saw you on my radar.
Paulo: But you don't have radar...