Diagnosis of injury to the meniscus is made after taking a 'history', doing an examination, and then requesting (if necessary) special investigations.
Although every meniscal injury is different, there is often a story of a twisting injury, followed by swelling of the knee and joint line pain. Commonly the injury occurs in the 'planted' knee - imagine a football player landing on the knee ('planted') while the other knee follows through with the kick, with the full body continuing the momentum and continuing to rotate.
There may be immediate sharp pain, and perhaps a 'pop!' and swelling. Later the patient may complain of persistent pain in the joint line, and episodes of catching or locking. The knee may be locked in flexion (bent) at the first injury and may need to be reduced in the operating theatre (OR).
During examination of the knee, the doctor will be alert for signs suggesting a meniscal tear. There may be swelling of the knee and focal tenderness along the joint line.
The doctor may also perform some specific tests, which may offer further weight to the suspicion of a meniscal tear -
X-rays are generally unhelpful for recent tears, although a standing X-ray may give clues when a meniscus is totally incompetent.