Lostgallifreyan wrote in
:

Vz equals peak-peak voltage minus
about 0.58V.

Evidently my turn to pass annoying through the same doorway...

Scratch that, the rest stands though, you just have to do the test once for
each polarity of that diode. (You'll only see both peaks at once if you have
a zero-referenced AC voltage, and even then that capacitor idea wouldn't
apply, and I made it more complex than it needs to be, you only need to see
the zener peak to know...).