Similar to phishphreek80's idea, you can try this low tech solution as well - assuming you have physical access to the switches, power one off (killing all network connections) - check your PC's and see which PC's now have no access to network resources (Internet, file shares, etc.) - record those, then repeat the whole process for each switch that you have.

As mentioned, if you know the IP addresses, usernames and passwords of the switches involved you can log into those switches (possibly via telnet, console, or HTTP access) and view the MAC addresses of the PC's connected to each port on the switch. This of course is all dependant on how much info you have for each switch and if those switches are currently assigned IP addresses.