Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Set Multimeter to Resistance Set meter to measure resistance
Disconnect flame sense lead from the electrode
Disconnect in line connector for flame sense lead (near gas valve)

Measurement:

Check for continuity (zero resistance) between the electrode end of the lead to the in line connector.
Check that the lead and connectors are in good condition.