Cheap Tool Length Sensor Options

[m_c]

I'd always recommend testing the probe prior to any automated probing routines.
If you're manually changing a tool, then you're already at it, so for all the time it takes to trigger the probe/touch the plate against the tool, it ensures things are working, and eliminates the risk of a broken wire, or more likely, you've forgotten to plug it in

[Muzzer]

If you fit a "loop back" connection within the probe plug, you can detect if the probe is(n't) plugged in and inhibit probing moves. Requires an additional input but equally it reduces the risk of a crash. Some probes also have a secondary (overtravel) switch that you can wire into the e-stop circuit in case the probe connection doesn't work.