Xorg Multi Session Single Monitor: where am I?

[Cagnulein]

Scenario:
- 1 monitor
- 2 XOrg session (one for user 'a' and another one for user 'b')

With CTRL-ALT-F7 and CTRL-ALT-F8 I can switch between these 2 virtual 
sessions.
But how can I know which session is on the screen? (and so which one is 
on background?)

I need this information inside a little daemon running as user 'c' 
outside XOrg sessions

Thanks
Roberto Viola