Unfortunately ACL (anterior cruciate ligament) injuyr is permanent ligament injury, though it is often initially diagnosed as a "sprain." Therefore, it is important to contact your knee doctor for a definitive diagnosis. Often, he/she will be able to provide you with a diagnosis based on your history and physical examination.
They may suggest an MRI, which is a special test that utilizes a magnetic field to image the knee. Unlike an X-ray, which shows only the bones, an MRI also helps image the details of the ligaments, cartilage, and muscle. Mechanical testing devices, such as a KT 1000 may more accurately measure the amount of pathologic knee motion.
No single test or study is 100% accurate, therefore your physician will use a combination of history of injury, physical examination, and possibly further diagnostic studies to make the diagnosis of ACL tear.