Click the link below, and you will be shown a specially colored page. If you have dead pixels of that color, you will notice them as black dots. There are seven modes: two for testing red subpixels, two for green subpixels and two for blue, and yet another one is black screen test which allows finding glowing pixels of any color.
It will be better to open your browser in full screen mode (F11 key) to cover entire display area.
In test mode, you will see nothing but colored space. Click or tap anywhere to switch to next mode. Switching will go in a loop. To finish testing, just hold your mouse pointer somewhere in the middle of the screen and you will see a button to finish the test. You can also click "Back" button in your browser (Alt+Left Arrow key).
If you need to test your mobile display for bad pixels, make sure you open test page at 100% zoom, because any down- or upscaling will distort the pattern, what can make it difficult to see black dots.