How to know if i have meniscus tear?

You can't: A medical professional should be able to give you a working diagnosis regarding likely or not likely a 3rd degree sprain, or full tear. Without MRI or arthroscopy, we can only be mostly correct regarding damaged meniscus or not. Being able to walk on that leg does not rule out torn meniscus, nor does the absence of swelling. The pain will eventually subside even with treatment.