How to know if someone is connected to your pc using ssh?

I've been using ssh command since January 2011, I use this to kill some unnecessary running application in our employees, also to view if they were running Firefox while in time of duty. There is a time that I use ssh command in a person who also know how to use the top command, where he can see that ssh is running on him. I want to know if there's a way he can tell that this certain pc is the one who is connected on his pc?
