Monitoring circuit for the switching voltage
The simplest and probably the most fool-proof method is to use a capacitively coupled isolating pulse transformer. This then allows you to take the isolated differential signal and reference it to your monitor circuit ground. In other words, the pulse transformer provides the galvanic barrier you might need and it also delivers a single ended signal that can be monitored by a voltage comparator circuit.
If you didn't require galvanic isolation, you can still use a transformer as per above or, you can use a differential amplifier made from an op-amp and four resistors.