If you are sure it's a Zener, it can be tested for what voltage value by connecting a power supply vis a resistor 1 to 2k ohms in series. To determine the voltage, it helps to know what kind of circuit you have.
Power supply must be higher than zener value and this for low voltage zeners only.
Measure the voltage across the zener. Bear in mind that zeners come in designated voltages like resistor values. They also have tolerances like 5 or 10%. Check the measured voltage to the designated values to determine nearest voltage value.