Danger, Will Robinson, Danger!
I used to love watching Lost in Space. On that show was a life-sized robot that would declare, "Danger, Will Robinson, danger!" when there was some danger. Unfortunately, no such robot warns of danger on our systems. (Although some tools exist, they are nowhere near as consistent as that robot was!)
If you have a lot of extra disk space, you can turn on auditing, which records all user connects and disconnects from your system. If you don't rely on auditing, you should scan the logs often. A worthwhile alternative might be to write a quick summary script that reports the amount of time each user is on the system.
Unfortunately, there are too many holes to block them all. Measures can be placed to plug the biggest, ...