To list all of the screen sessions for a user, run the following command as that user:
screen -ls
To see all screen sessions on a specific machine you can do:
ls -laR /var/run/screen/
I get this on my machine:
gentle ~ # ls -laR /var/run/screen/
/var/run/screen/:
total 1
drwxrwxr-x 4 root utmp 96 Mar 1 2005 .
drwxr-xr-x 10 root root 840 Feb 1 03:10 ..
drwx------ 2 josh users 88 Jan 13 11:33 S-josh
drwx------ 2 root root 48 Feb 11 10:50 S-root
/var/run/screen/S-josh:
total 0
drwx------ 2 josh users 88 Jan 13 11:33 .
drwxrwxr-x 4 root utmp 96 Mar 1 2005 ..
prwx------ 1 josh users 0 Feb 11 10:41 12931.pts-0.gentle
/var/run/screen/S-root:
total 0
drwx------ 2 root root 48 Feb 11 10:50 .
drwxrwxr-x 4 root utmp 96 Mar 1 2005 ..
This is a rather brilliantly Unixy use of Unix Sockets wrapped in filesystem permissions to handle security, state, and streams.