Drag the window inside watching it closely as it moves from top to bottom and left to right on different parts of the screen. Try dragging it at a moderate pace, as shown below.
It is advised to lower your brightness of your monitor and run this test in a dimmed room. Change size of the window, its border thickness. Try different colors of window, background or border to get the most reliable results. For most cases using presets will be enough. For short test use the default 50% gray tone.
Blotches, discolorations, luminance deviations are visible especially when uniform elements are in motion. Moving them around the screen makes them appear to human eye even when they are not visible on static color on full screen.