detecting when a user start a gnome session

[leonel florin cfg jovenclub cu]

Hi friends, I'am trying to develop a management tool for admins, and one of the
thing I need to do is detect when a gnome user start a gnome session, I use the
users command line, but it give all the users that are log in , but I only want
those one that have a gnome session up or X session up.

do you know how to do that. which command line should I use