Basically you have 2x sources....
*The router.
*A managed switch.
As for the router....depends on the router. Some have this feature built in...typically not residential grade routers, not many SMB grade ones either. But ya never know. And if it's a router that supports some 3rd party firmware...like Tomato or some DD flavors...yup....you can get some with MRTG or PRTG built in. Many *nix UTMs have a more advanced feature. There are some good plugins for PFSense, and I know Untangle has a great bandwidth monitor (a pay for add-on..but you can get a free trial)
A managed switch would also help you find this....if you know what port each client was plugged into.
There are some free software products which you can run on a computer for a while....or at least free trials.