Dead Pixel Test
Six fullscreen solid colors, one after another. Wipe your screen first — most 'dead pixels' turn out to be dust — then inspect every slide corner to corner.
White
Black
Red
Green
Blue
Gray 50%
Click or press any key to advance through the colors; Esc exits at any point. Inspect the whole screen on each color — a pixel that stays a different color on every slide is stuck or dead.
A dead pixel stays black on every color — its transistor gets no power, and it's effectively permanent. A stuck pixel shows one constant color (red, green, or blue) because a single subpixel is jammed on; these sometimes recover on their own or after a few hours of rapid color cycling. The black slide doubles as a backlight-bleed check — uneven glow along edges in a dark room. If you find defects on a new monitor, check the warranty quickly: most manufacturers replace panels above a small threshold of faulty pixels, and some (and most retailer return windows) accept even one.