control alt del for task manager, go to networking. add the appropriate stats you want to see under View and also go to View->Select Columns and add in Bytes Received per pnterval and bytes sent per interval. Set the update speed to high and mulitply the numbers you see by 2 to get the number of bytes per second being sent/recved.
If nothing's using up your bandwidth while you're idle, it's not using it up while you're playing tribes.