The doctor and/or the therapist may be using two tests of joint motion to look for joint laxity and joint stability. One is called the Lachman test. The other is the pivot-shift test.
Most of the time patient satisfaction and return to previous level of athletics or other daily activities are the most commonly used measures of success.
Does the knee or leg ever give way underneath you? Have you had to have a second or third operation after the ACL repair? Do you have any knee pain and/or swelling? Any of these problems may be a sign that the surgery hasn't been a success.
In some physical therapy clinics. Special machines are available to test your strength through the available range of motion under different load levels. Expect to see gradual increase in strength over time as a measure of success.