Set Appliance State:

Set Multimeter to Resistance Set meter to measure resistance
Isolate Mains Power to appliance as per TOPs P4
Remove the burner to gain access to the spark electrode
Remove the spark electrode and lead assembly

Measurement:

Check the electrode and its lead for signs of physical damage, cracked ceramic or insulation.
Also carry out a resistance check from the tip of the electrode to the spade connector at the end of the lead.
Expect continuity.
Note:
Before reassembly check the integrity of all disturbed gaskets and seals and replace as necessary.