I'd like a simple method for this as well.
The best I have so far is to access DarkStat and then hosts.
From there I just refresh to see what's doing the most bandwidth.
From there it's tricky though, sometimes I can figure it out just from the device, but if it's my mac, I have to use something like Little Snitch to see what process/port is being naughty.
If someone has a simple solution, I'm all ears.