How can you tell if the walls of an empty inground swimming pool are level?

You can use a transit by placing it in the shallow end of the pool, rotating it around and have a friend with a stick walk around the edges of the pool, comparing the mark on the transit sight to the mark on the stick (mark on stick is made with marking height of wall on stick). For more on this, check out the instructions on diy inground pools. Transits can be rented from tool rental houses. The 2nd method would be to get a 360 degree gyroscope laser lever, wait until night time and again set up in the shallow end, prop laser level up on a box or piece of wood until laser line is level with top of wall in one spot, should be level with wall in all locations if truly level. These can be rented from tool rental places as well.