Set Appliance State:

Set Multimeter to mVolts DC Set meter to measure DC mVolts
Isolate Mains Power to appliance as per TOPs P4
Gas Control Knob in Pilot Ignition position
Gas On; Pilot On

Measurement:

Measure the voltage across the Thermocouple spade terminal whilst keeping the Gas Control Knob depressed. The Voltage measured should be greater than 12mV DC.