Is there a reliable way to tell if a registered user is logged in?

The reason I need this is that the DNN site has a sister site that they may be passed over to during the visit. Once they get to the sister site, I need to verify that they are actually logged in on the DNN site. The userID is passed over to the sister site, so I would use that to make a database call which would tell me if the user is logged in.

The only thing I have found so far is the "Last activity date" in the aspnet_Users table. The "UsersOnline" table didn't appear to actually be updated when a user logged in.