How do i know if i tore my acl?

Get examined: The best and most reliable way to know is to have your knee examined by a sports medicine doctor.

A good exam then MRI: Although most people think a MRI is necessary to diagnose a torn acl, most knee specialists can tell from a couple of simple test that can be done. With a high suspicion, a MRI is often then ordered and used then to confirm the findings of the exam. My suggestion for you is to see a md who is well trained and experianced in acl's and even revision surgery. Get their opinion.

Instability : Acl tears usually cause significant swelling, pain, and feelings of instability ...go see your doctor for evaluation and possible mri

History and physical: Would recommend an orthopaedic history and physical to see if further workup including and MRI to make the definitive diagnosis. For an ACL tear, one most often has a significant injury to the knee with an immediate filling of the joint with blood. Chronic symptoms of ACL tear are giving way of the knee and a persistent effusion.