Set Appliance State:

Gas On; Pilot On
Set Multimeter to mVolts DC Set meter to measure DC mVolts
Isolate Mains Power to appliance as per TOPs P4
Selector Switch OFF
Gas Control Knob fully depressed.

Measurement:

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