Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Gain access to PCB (use Insulation Mat as required)
Disconnect white leads at the thermostat

Measurement:

Measure the resistance between the disconnected leads at the thermostat.
As boiler is up to temperature resistance should be open circuit (1MΩ or O.L).