Set Appliance State:

Mains Power Off
Set Multimeter to AC Voltage Set meter to measure AC Volts
CH Demand
WARNING! Mains voltage is expected during test, follow TOPs P4 live testing procedure
Mains Power On

Measurement:

Measure the Voltage across the Room Thermostat\External Controls terminal marked ON and the neutral (N) (Blue wire on the main connector block).
Note:
If a Room Thermostat\External Controls are not fitted there will be a Link Wire connected between the connections marked ON and HW.