how to detect downstairs or cliff using pointcloud

edit

hi, I am planning to make a roomba like robot using lidar and depth sensor. I used pointcloud_to_laserscan for 3D object detection but I am stuck with ideas on how I can detect cliffs or declined surface. Though I came across this depth_nav_tools package but looks like it works along with costmap. for my application I am not creating a map and I am just letting the bot navigate in an unknown environment with downstairs in it. Any insights on this would be really helpful. Thank you.