Voltage rating of a zener diode found out of nowhere

What precisely do you mean by the "Voltage Rating" of a Zener diode? If you mean the Zener Breakdown Voltage, then you should use that term. If you don't have a part number to connect with a datasheet, then testing is your only recourse. I seriously do not know who could inspect it and tell you anything about the device.

If you mean the "Zener Breakdown Voltage" then put it in series with a 10k resistor and measure across the diode.

BTW, how do you know it's a Zener?

Mike.