Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to mVolts DC Set meter to measure DC mVolts
Gas On; Pilot On

Measurement:

With the pilot alight (keep Gas Control Valve Knob pressed in) measure the voltage on the Thermocouple between the interrupter and a suitable earth.
Voltage measured should be greater than 8mV DC.