A simple way might be to say 'is this user currently loggedin, and not logged out' - and this is recorded in moodle already. But if the session times out I don't know if it records the logout - as opposed to them clicking logout explicitly and it tidying up cookies, sessions etc.
A harder core way would be to capture a unique identifier for the workstation the user is on (something like the MAC address for the network adaptor on their machine) and save this is in the mdl_user table. Then check for this being present / compare it to any future login attempts.
But I doubt a web browser can see this information. Unless you start faffing with browser plugins / activex controls on the login page - which would have to be either cross platform or autodetect/support several platforms (safari, firefox, IE, different OSes etc).
Another developer might know of something more elegant, or advise better on the feasibility of my rough suggestions.