What is a common visible sign that a brake may be out of adjustment?

Correct answer to question 116 (Air Brakes): The slack adjuster arm forms noticeably more than a 90-degree angle to the pushrod when the brakes are fully applied

Why

When the brake is applied, a correctly adjusted slack adjuster forms about a 90-degree angle to the pushrod; an angle noticeably greater than 90 degrees means the pushrod stroke is excessive and the brake needs adjustment.