[Xack]

LED's are diodes. They emit light when forward biased. They don't work or conduct when reversed biased. In other words, put in backwards they don't work. You can see if they are OK, by using a VOM with a diode check. Normally this is the diode symbol. The VOM will beep when red is on the Annode or the back of the triangle and the black is on the cathode or the line across the tip of the triangle. If you reverse the connection then nothing will happen.

A diode is a one way valve. It will conduct in one direction. You reverse the connection and it acts as a switch turned off. They are pretty simple and usually use much less current to illuminate vs. incandescent bulbs.