testing diodes       

 
Testing a diode is relatively simple if you have a digital multimeter.
Set the multimeter to diode test and connect the diodes for forward bias .......

>black probe to
cathode ( - )

and

> red probe to
 anode (+)

the meter should show (around)
 

1.8 volts for LEDs
 this will vary with the different colours!! as the different semiconductor mixes have different forward voltages

OR

0.65V for Silicon diodes

OR

0.2V for Germanium diodes (use germanium as a semiconductor substrate)

 its that simple!

If you find readings that do not indicate a healthy diode it has probably been overloaded!!
Its always best to test an electronic component out of the circuit.

If you do not have a digital multimeter but have an analogue meter then check forward and reverse resistance using the ohmmeter.