Diagnosing osteoarthritis usually involves a doctor pursing the following steps:
Taking a "history of the present illness," which includes reviewing what symptoms have been occurring, for how long, and under what circumstances
Reviewing the person's medical history and checking for other medical conditions that could be related to the joint complaints
Conducting a physical examination, with a special focus on the affected joint(s) and body areas
Taking an X-ray of the affected joint(s), to look for signs of joint wear or damage
Additional tests, such as blood work, may be ordered if the doctor thinks they're needed to rule out other causes of the symptoms. Unlike certain types of arthritis, such as rheumatoid arthritis, there's no particular abnormality on blood work that's linked to osteoarthritis.
Occasionally additional testing is recommended, such as sampling the fluid inside the joint or ordering an MRI. However, in most cases, these steps aren't needed.