Inspect the screen of the laptop and determine the location of the viewable area. The viewable screen area is the part of the laptop screen that displays information and is illuminated. It does not include the casing.
Take a tape measure, and carefully measure from one corner of the viewable screen area to the opposing diagonal corner.
To be sure your measurement is accurate, measure again using the opposite diagonal corners. Laptop screens are rectangles, so the measurements should be exactly the same along both diagonals.