Transconductance (gm) is measured in siemens or amps per volt. (It is like the inverse of resistance). A circuit like this can be used to find the value of gm.
The potentiometer is adjusted to give a range of input voltages (Vgs) and corresponding drain currents (Id). The drain current Id is measured for each gate voltage. Be careful not to fry the device with excessive currents! Plot a graph of these measurements. It should be fairly straight but the drain current will not start to flow until there is a significant positive gate voltage (maybe 2 to 4 volts).
The gradient of the graph (amps / volts) gives you the transconductance.