I found out the easy way to test any zener diode is simply to hook it up to an old style "eye type" capacitor tester.

Remove the zener from the set, and hook it up to the test terminals of the capacitor checker, along with a meter in parallel with it.

Place the capacitor tester on the "Voltage Test" position,
and crank up the variable voltage. When the voltage measured across the zener will not read any higher, no matter how much you crank up the test voltage, you have the operating voltage of the zener.

Great for testing those unknown zeners you get in a "bargain pack", unmarked, or suspect ones in a set.

The old "eye type" capacitor testers have a built in current limiting resistor, so the factory smoke will not come out of the part.

I have used this method for decades and it has been 100% accurate so far. I even built a test jig with test clips on it that plugs into my cap tester, with jacks for my meter to plug into.

I've done this also, and also I've tested various "neon" bulbs, some glow other colours at different voltages, as they use different gasses.

You could also determine PIV of diodes, but you want to be sure the leakage current is very low (say a few megohms resistor between the diode and the tester) or else you find out out what the diode used to be able to do...