[Radio Wrangler]

Most of these suggestions have to carry the qualification "probably"

All diodes break down into an avalanche mode at some voltage. There are some zeners made with quite high breakdown voltages and there are signal and switching diodes made with breakdown voltages as low as a few volts. So there is a lot of overlap between the voltage range of available zeners and that of other diodes. Most commonly used ordinary diodes have higher breakdown voltages than most commonly used zeners. Just be wary that there are enough exceptions to catch you out occasionally.

In a purpose-made zener diode the processing is controlled to set the breakdown voltage - usually close to an E12 or E24 value (at their specified current, and that throws in another unknown!). The processing is also designed to remain stable over a long period spent in breakdown. Ordinary diodes will degrade under this treatment. Intentional zener diodes are designed to show a sharper knee in their characteristic. These aren't hard boundaries that can give certain identification of zeners so there's no certainty here.

Not all zeners are used as reference voltage sources, they get used in signal circuits to offset signal voltages or to clamp them, so they can be found floating around away from ground potential.

Some of the signal diodes with low breakdown voltages can be rather valuable. Tunnel diodes, microwave noise sources and microwave detector diodes. Not commonly met but can be damaged by an ohmmeter test.

The only sure identification comes from either reading a type number, or seeing the schematic.

Measuring the breakdown voltage gives probable identification, but you should at least be aware of exceptions, otherwise when you meet one it will really lead you up the garden path.

David