Set Appliance State:

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

Measurement:

Measure voltage across thermocouple whilst keeping Gas Control Knob depressed, with the pilot flame alight.
Voltage measured should be greater than 12mV DC.