Should be able to read 5v every time the magnet passes the sensor from the ground lead of the hall wires, but it's not a guarantee of correct function.

You can also hear the snap Jody was referencing if the ignition is arcing to ground at the outside of the cap. Basically going from the end of the wire to the metal wall inside the cap, bypassing the plug completely. Then you have the condition where you have a cracked insulator ceramic with a spark plug where the electrical arc travels down the outside of the plug and grounds to the hex base of the plug.

Or have a loosely mounted cap where the spark arcs from the wire to the top of the plug, causing intermittant firing. That generally leaves black carbon traces on the upper portion of the plug insulator. This is very common for users of plug caps used on RC Exl ignitions because the user fails to push the cap on securely.