You can use almost any white LCD screen and a polarized filter to determine if a sheet of glass is tempered.
Most LCD screens have a polarized filter set at a 45 degree angle across the screen. This is why when you look through polarized sun glasses and tilt your head one way(poarized filter on screen and sunglasses are lined up) the screen gets brighter and when you tilt your head the other way (polarized filter on screen and glasses are perpendicular)the screen goes almost completely dark. This will work with almost any LCD screen...small cell phone LCDs as well as Laptop LCDs. A very few LCDs do not have this type of filter. If it goes dark when you tilt your head one way or the other while wearing polarized sunglasses then you can use it to test your glass.
This works because the stress in the tempered glass causes light to polarize in a pattern within the glass. Some people can see this as "color patterns" when sunlight passes through tempered glass. I have never been sure what to look for and so could never be sure if glass was tempered. A clear way to see if glass is tempered is to place a white LCD screen (notepad maximized) behind the glass and look at the Screen through a polarizing filter.
If the glass is tempered it will look like this view through the bottom:
If it is not tempered it will look like this view through the side panel: