Find a spot on the railing that has a support or balustrade. Measure straight down from the top of the railing 12 inches along the support with a tape measure. Make a mark on the support with a pencil.
Place the 90-degree angle intersection of a carpenter's square at the point you measured to, with one leg of the square extending vertically upward toward the top of the rail and the other pointing horizontally toward the downward run of the rail.
Have an assistant hold the square in its position. Place the end of your tape measure at the mark you made earlier and measure outward from the inside of the angle of the square to the top of the railing, measuring along the horizontally placed leg of the square. Record this distance in inches on a piece of paper.
Divide 12 inches by the second figure and multiply by 100. For example, if the second measurement was 24 inches, (12/24) x 100 = 50. The slope is 50 percent.