To definitely differentiate black zeners diode

[abdulbadii]

How do we differentiate black zeners like below with that of a diode ?

[nostril]

There are a number of options, including:

1) checking the part number written on the component
2) checking its position in the context of the rest of the circuit
3) measure the component's zener/avalanche voltage
4) ask the person who designed the circuit 

Hope that helps

[Gyro]

How do we differentiate black zeners like below with that of a diode ?

I wish you would stop it with the lazy one line cryptic questions.

[jonpaul]

curve tracer
lab power supply and
VOM
j

[Audiorepair]

Put your meter in diode mode.  Zeners nearly always measure in the 700mV range, standard diodes in the 500mV / 600mV range.
Other diodes like Shottky types can measure way lower.

You can do this with the diodes in circuit.
Try it, you will usually easily spot the zeners.

[Simon]

some context to your question would help