If an engine was running well, then suddenly stops running well or deadsticks for no other known reason - change the plug.

If an engine starts okay, but then drops in RPM or stalls when you remove the glow igniter - change the plug.

If you take the plug out and it looks damaged - change the plug.

If you take the plug out and put it in the glow charger and it doesn't glow - change the plug.

Brad