Comment 6 for bug 875817

Consolekit is exactly for this purpose; tracking users, logins, and seats. You can read a little bit more about it at http://www.freedesktop.org/wiki/Software/ConsoleKit

It reports all of this through modern mechanisms; dbus. Basically any interactive (local or remote) session will invoke a console kit session. This is why you'll see things like SSH listed in it as well as X.

ck-list-sessions is actually a wrapper tool around it's dbus interface. It will show you lots more information than you probably care about, but it can certainly tell you whether a user is logged in or not. That tool itself might not be well documented, but that's because it's output is pretty self explanatory. You can read it's code here to understand more if you're a bit confused still:
http://cgit.freedesktop.org/ConsoleKit/tree/tools/list-sessions.c

I think the best thing for you to do though is read the documentation. The DBUS api is documented there and you can take advantage of it. http://www.freedesktop.org/software/ConsoleKit/doc/ConsoleKit.html