Break a hole in the scum layer with the pole. Press the towel end of the pole down through the liquid until resistance is felt. This is the sludge layer. Press the pole down through the sludge until the pole reaches the bottom of the tank. If the sludge depth is one-third or more of the liquid depth, cleaning is needed.
Some authorities recommend cleaning when the sludge becomes one foot deep.
To check the scum level, use another pole or 2x2 stud about eight feet long. Nail a flat board about four inches square to one end. Push the end with the board attached through the scum layer and move it to one side into an undisturbed scum area. Lift the pole slowly until resistance is felt from the square board contacting the bottom of the scum. Put a mark on the pole even with the top of the access hole.