How could a person be diagnosed with a knee cartilage problem?

Hx physical mri. Pcp should be comfortable with this work up amd dx.
Exam, MRI etc.. Symptom pattern, history of trauma, genetic factors, age (older, more likely osteoarthritis etc...), disease such as rheumatoid arthritis etc...Will influence the chance of having cartilage problems. Xray may show narrowing of joint spaces which suggest so..Mri is a more definitive test. I would consult doc for an evaluation and go from there... Good luck.