1. Set FOV to "Quake Pro" (Or 110)
2. Shift (or whatever your crouch key is) onto the VERY edge of a ladder
3. Aim upwards towards the center or left-center of the ladder, as well as about halfway to the top of the screen. You should slightly see through the walls.

I've included an attatchment / replication of how to do this. As you can see, I can see the moon (on my resource pack) through the wall, when I shouldn't be seeing it at all.