How to access screen created by other users?

Unless the screen session in question is created with multiuser on, you can't. Even if you set your SCREENDIR variable to point at the other user's socket directory, screen will just complain that you don't own the directory and quit when you try to use it.

Of course, you can simply su to the other user and use screen in the normal way.

You can see all screens on your machine via:

ls -laR /var/run/screen/

This will at least list the sockets with screen sessions and the user who owns it. If you have permissions to the socket it will list all the screens in use by that user as well.

from here