There are three different types of defective pixels:
- Hot pixels (always on).
- Dead pixels (always off).
- Stuck pixels (one or more sub-pixels are always on or always off).
Defpix is a free program that can help you with checking for defective pixels on your LCD screen. The program simply shows these colors – red, green, blue, white and black – over the whole surface of the screen. Defpix requires no installation and supports multiple monitor systems.
Here is a close-up of an LCD, showing a dead green.
To learn more about dead pixels check out this article in wikipedia.