How do you test a slack adjuster?

Assuming we are talking about "S" cam air brakes on a truck, check the angle at the end of the slack adjuster to the actuator rod coming out of the air brake can when brakes are applied, it should be 90 degrees, if the slack adjuster is manual adjust it with the brakes not applied and then test again. If they are automatic slack adjusters and you do not have 90 degrees with brakes applied it is worn out or not installed correctly.