Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect DHW flow switch inline connector

Measurement:

Measure for continuity across the Black and White leads of the disconnect inline connector as follows.
TEST 1. Turn DHW tap on, continuity should be measured.
TEST 2. Turn DHW tap off, open circuit should be measured.
Note.
Repeat this test several times to confirm correct operation, as the DHW microswitch is known to stick.