NETWORK SLOW

Need help in troubleshooting our a branch network

We are an organization with 7 branches. One of our branches has been having regular slow network connection on the LAN side. Users cannot access our resources at the head office especially the system. this has hindered the services occurring in that branch. We have 2 cisco switches connected to each other with two links and users connecting to them. I suspect there is something sucking up the bandwidth. cause our wan connection is 100% at normal operational bandwidth.

1. I need help in troubleshooting, to narrow down the machine that sucking up the bandwidth if it is a loop, how to I pinpoint the device that is causing so. can software like colasoft Capsa help in identifying the same?
2. also, what best practices do I need to configure the switches to make them more efficient. we have two 2960 series switches.

will much appreciate any output.

Regards