How to determine which points lie in viewbox after zoom in

Hi all ...
 
what do i do to determine as to which points are visible in the viewbox on zooming in .
I have to do different actions based on the point coods visible in the viewbox.since zoom in depends on the portion selected for zoom in....how do i determine which all points are visible after having zoomed in?
 
Any help would be of great use....
 
thanx...
regards...
Kalpna