How to get list of active user logged on to the system with terminal and other details

Use the T-code SE37 and execute the function module TH_DISPLAY_USER_LIST.

Here you wil get the list of active users with information like Client, Terminal, Transaction code the user is working on, number of sessions opened by that user and whether Trace is ON or OFF. 

This information can be used for sort of troubleshooting purpose.