It takes some practice.

We were taught that if you put your nose right in the center of the image, and then let your focus go, and pull back from the image, that at a certain distance from the image (as your focus is still at ∞) various structures of the image will begin to resolve. So contrasts, similarities, and shades will all resolve at different focal lengths from the image.

It was rare that any one person would be able to pick up immediately upon all the effects perceptible in an image. I was able to pick up on certain shades of the color green that are used in contrast to red, but it took me a long time to get the shading of black-white (as in this optical illusion - and it is but one of many).

When we were tested on this, we would not be told what was similar, or where optical tricks were used, and we would have to pick them out of an image (and this was long before the internet, so we couldn't just go online to do a search for optical illusions to find images to study that had their illusions spelled out for us). So, it is a skill that can be learned. For me, eventually I had to learn how to focus upon each square with a different eye, while squinting, and letting the focus go back and forth between my right and left eye. eventually, I get the images resolved as a single shade as I go back and forth between my eyes.