diode

Construct a simple circuit with a cell, ammeter, resistor and the diode under test.
Test both directions of the cell connection.
If current flows in both direction or does not flow in both direction, then the diode is defective.
Normally, diode only allows current flow in 1 direction (forward bias)

The most simple way is to use a multi-meter to measure its resistances in the forward and reverse bias directions.