There are these handy stored procedures to list the current (and currently executing) sessions in a SQL Server database.
sp_who: Comes pre-installed. Lists session currently connected to a database. More info here.
But typically people prefer sp_who2, which displays more information.
sp_who2: Comes pre-installed. Lists session currently connected to a database. More info here.
But to easily see the active sessions, and what they're currently doing, you want sp_who3.
sp_who3: You need to install this yourself. (Definition is here.) Lists only sessions with a currently executing request, along with what that SQL statement is.