I spent far too much time as a twenty-something generating autostereograms, which seems to have trained my eyes. I was able to "cross" the images on this page very quickly.
NB autostereograms require you to move your eyes away from each other, the opposite of crossing them. To put it another way, crossing your eyes is what your eyes do when you're looking at something close to you, while the opposite is when you're looking far away.
Which is why for ASGs people advise you to look past the picture. Or why you bring the pic close to your eyes (so close that you basically have no choice but to look beyond the picture)
Ever since I was a child addicted to the "magic eye" stereogram books, I've always diverged (not crossed) my eyes for spot-the-difference puzzles.
Also, if you're doing it on a piece of paper, hold a pen in each hand spaced right so you see the middle (3rd) hand in the middle combined image, and move both hands in sync to circle all the differences. Kind of a cool way to point them out to someone else.
The difficult puzzle took me about 10 seconds here since I was looking for more than one difference. I saw the first difference in about 1 second.
You can easily generate inverted ones that require crossing your eyes to appear properly, but they don't look as nice since they pop out instead of going into the screen/book.
