There is a simple way to measure the height of a basketball hoop. However, most people don't know the proper way to check official basketball rim height for the sanctioned games.
To check the true height of a basketball rim you will need:
A tape measure
Have one person climb the ladder and place one end on the tape measure at the front tip of the rim so it is even with the TOP side of the rim. Drop the tape measure straight down to the playing surface to check the distance. The tape measure should read 10 feet for regulation basketball, typically ages 11 and older. Next perform the same steps at the back side of the ring. If one side of the ring is off the 10' mark, you will need to make adjustments to the rim or backboard to compensate until perfect 10' height is achieved front to back on rim.
Shown below: Measuring basketball hoop height