How to tell who is logged in to SQL Server

Question

I am designing an application and I'm looking for the best way to tell who is currently logged into SQL server. I will use the Activity Monitor built into SSMS, which shows a list of all processes accessing the server - very cool.

However, if in my .NET code it uses a separate connection each time I access the database then how will I be able to tell who is currently logged in? If a user is just looking at a screen and not retrieving data from the database at that moment then they wouldn't show up correct?