Diode and LED Tester
The simple Diode and LED Tester presented here costs under £5 to build (including all components, the battery and a cheap case) and will indicate the polarity of almost all types of LED and other diodes with no risk of damage. The average test current is about 5mA (10mA pulses), which is sufficient to illuminate the LED being tested.The unit has two test probes and two indicator LEDs. The diode or LED to be tested is simply connected either way round between the test probes, and the cathode connection is indicated by the LED closest to that connection illuminating.
   Diode and LED Tester
Read more on Diode and LED Tester website
www.electronic-projects.net