There are two components to the nabi screen, the glass/touch panel which recognizes touch commands, and the LCD which displays images and video.
Glass/Touch Panel: this is a piece of glass and even though glass does not just break by itself, it can crack with excessive force applied to the surface of the glass/touch panel.
How to tell if this is damaged: the easiest way to tell is if the top layer of the screen is cracked or shattered, but the images on the screen display normally. Please note it is possible to damage both the glass/touch panel and the LCD.
LCD: the LCD is made of liquid sandwiched between two pieces of glass. This can crack as a result of excessive pressure, twisting of the device, or extreme heat.
How to tell if this is damaged: You may see inconsistent lines and patterns on the screen. Also, black "spots" may appear. This can be the liquid crystal material spilling out of a crack.
Please note: repeated bumps, drops and mishandling may weaken both the glass/touch panel and LCD, allowing a minor impact to result in screen damage.