Ben , a good test to find the Iss of a fet ,

Very sensitve devices , I sometimes felt , that even blowing over them , would creat enough static to damage them

But , for an FG35 hempt , out of any circuitry ...........

Link the gate to the source .

Connect carefully , with short Wires , to a low voltage variable power supply , a 3 Volt battery is ideal with an adjustable output voltage  ( an LM317 )  and multiturm pot , and a mA meter in the output + line .

Set the output to Zero .

Connecy Positive to the Drain , and Negative ( common) to the emitter .

Slowly increase the Positive supply ........ at some 600 mV , the mA should be indicating around 50 to 90 mA

and at One volt , DO'NT GO HIGHER ,     the current should stablise at 80 to 110 mA

You have a good idea of your Idss ......

Turn the voltage back down to Zero , before removing wires .

For low noise Fets  60 to 80 mA

For medium power amps , 80 to 150 mA

Used to do this on a little jig , to select Fets used in a multstage amplifier ...... it worked very well and made alignment a breeze ..............