Once you've installed Rokario's free Bandwidth Monitor 2 Lite Edition on any PC on your network, it will track bandwidth use in real time. The program displays download bandwidth use, upload bandwidth use, and total bandwidth. It also keeps logs of bandwidth use, so you can retain a historical record.
Install it on every PC on your network and then check the results, especially the historical records of bandwidth use, regularly on each PC. That way, you can see whether any bandwidth hogs are wallowing on your network. Based on that, you can try to reduce its excess demand... maybe too much YouTube?