Doctor will identify torn meniscus by the history or the mechanism of injury that patient gives and does some physical examination tests which involve moving your knee and legs in different positions & localising the painful spots of your knee, finally conforms it by MRI scan of your knee.

Usual symptoms of a recently torn meniscus are pain and swelling of knee that develops over 24 hrs after injury

Other symptoms of torn meniscus are locking of your knee, difficulty in straightening of your knee