FPS Character controller crouch sneak

This one's a bit hard to understand unless you've played Unreal or Minecraft, but here goes anyhow:

In both of these games, if you crouch/sneak you cannot walk off a ledge that is at 90 degrees or more. This allows you to "feel" out the ledges without having to look down at them; something that is crucial to the type of game I am attempting to create.

Any ideas on how to approach this?