If you boot your console with the Setup key held down, this will boot you into Test Mode.
Test Mode will display a virtual Front Panel of your console. Moving faders, and pressing buttons will be highlighted on the virtual front panel, for you to check faders, buttons and encoders are registering correctly.
Pressing keys will light the LED in that button.
For consoles with 7-segment displays, these will cycle to ensure all parts are displaying correctly.
To exit Test Mode, restart the console. To do this switch the console off and on again.