Move the mouse over the red square below on a retina display. The expected result should contain alternating black and white pixels, but on Chrome right now it just looks like a single big gray square. Note that you need a retina display for this to make sense.
This test uses two images of black-and-white checkerboards, each with single pixel squares:
The custom cursor uses the 32x32 image for normal displays and the 64x64 image at half size for retina displays.
64x64 shrunk to 32x32: