This is a very simple utility so that you can check if your device is overscanned.
It shows little arrows that should just kiss the edge.
If your overscan is greater than the number the arrows will be blunted.
If your overscan is less than the number you start seeing arrows from the outside.
Just play with the buttons until it's right, that number is how many pixels are hidden by the bezel.
http://www.temblast.com/android.htm
This screen grab shows the outside arrows creeping in.
Visually, you can't see that.