How could you tell if you torn your meniscus?

Depends... Many people with torn menisci have no symptoms at all... Most do not experience much discomfort with walking... A few do. The most painful activities typically associate with a meniscus tear include kneeling, squatting, pivoting and twisting. Some may also experience giving way or locking if the piece is torn piece is large enough.
Examination and MRI. Some experienced Doctors can reliably diagnose a meniscal tear from physical examination, and at times the history of how the injury was obtained is very suggestive. In today's medical environment, most Doctors wish to confirm their diagnosis with MRI to see if surgery is indicated. Some insurances will not pay for surgery unless there is an abnormal MRI.
Mri. Diagnoatic arthroacopu knee arthrogram completes list last two of historical interest MRI jas replaced other tests.