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.

Related Questions

How do I know if I tore my ACL when I fell on my knee?

Acl. Immediate swelling loss of motion and pain are indications that u injured yr knee. An exam by an orthopedist would best answer this.