I think i either tore my acl or pcl. how do i tell for sure?

Mri: The only way to tell for sure is with an mri.

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 experienced in acl's and even revision surgery. Get their opinion.