See if you can throw a football.
I tore my ACL (and partial tear of LCL) playing basketball. After a couple of days I went to the doctor. He did all the standard manipulations to see if the ACL was loose, and it did not appear to be (this was before MRI's were common, and my legs were exceptionally strong back then). I skied and played tennis on it just fine, though it felt a little strange. However, one day I threw a football to my son. I fell over. I tried again. I fell over again. Something about planting the front leg and then twisting over it when throwing a football just could not be done with the torn ACL.