Troubleshoot over utilisation on office network

I have a cisco 1941 router coected to a switch, and a modem. I suddenly noticed a huge slow down with the inteet coection. It happened very abruptly. I did an isolated test and noticed i was getting the usual bandwidth from the ISP but when coected to the LAN, the coection slows down drastically. It is crawling. I suspect over utilisation from the LAN, maybe from a user but how do i check where the traffic is coming from? Please any ideas on how to troubleshoot this? Is there a way to check the bandwidth consumed by each client coected to the network?

Thanks.