I am using a RB750GL as a switch, where one port is the master port and ports 2-5 are configured as slaves. As a result I can only see the traffic going in and out of the master port (Interfaces List in WinBox), but I can't see how much traffic is transmitted on the individual (switch ports).

Basically I want to determine the amount of "load" (of the available theoretical 1Gbit/sec) that is going through the switch.

Release the ports from switch and set all ports to bridge. It will utilise cpu so it can limit the transfer speeds but you will have the statistics.