maybe: use a child cube, about a quarter of the height of the parent (so it doesn't collide too high up), with a length x width error-margin you prefer around the outside, that looks for collisions. When it's in collision with an object, you're (probably) on the ground.
Edit: oh, but make sure the bottom of the child cube is aligned with the bottom of the parent cube and offset downward by the error-margin.
