Set Appliance State:
Gas On; Pilot On
Set Multimeter to mVolts DC 
Isolate Mains Power to appliance as per TOPs P4
Measurement:
Measure the output voltage of the thermocouple by inserting one probe within the interrupter assembly to contact the thermocouple inner core and place the other probe to earth.
Isolate as per TOPs P4
and refit as necessary