How can I find the switch on my network here at work?

I am trying to monitor network traffic to see who is using the most bandwidth (the internet gets very slow sometimes, I know someone is streaming/using BitTorrent) but cannot seem to find the network switch. I have tools like nmap, ettercap, wireshark, and what not, but how can I use these to locate the network switch? Is it just a matter of scanning the whole network and finding the default gateway? Or is the switch not the default gateway and a completely separate router/machine?

Any help is much appreciated!

Thanks!