Using a multimeter. Try latching the lever into manual and create a heating demand, if water goes to cylinder as well then it’s likely to be the zone valve
Open the hot water valve manually - make sure the little lever is hooked in so it stays in the open position - there is a little indent where the lever locks in. Then turn on the heating by raising the room thermostat - this is so the boiler comes on. Then feel the pipes on the HW valve - should start getting hot. This will heat the HW tank until you can get the valve fixed.
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