It's always quite easy for me to see them, since I have a book full of those. After a while, I can even see 3D features in tablecloth, bathroom floor tiles or anything else with repetitive patterns .
If you read those patterns following the same way you read those 3D-pictures, you can see the patterns get much larger and two layers of them appear . (No, I was not smoking anything when I did it.)
Give it a try when nobody else is around.