There is no single test that tells you that you've got osteoarthritis. Instead, your doctor will ask you about your symptoms and medical history. He'll also do a physical exam to see how well your joints work and may order an X-ray or another imaging test.
See your doctor as soon as you notice unusual symptoms so you can get started on treatment. Your primary care doctor may suggest you see a rheumatologist, an expert in joint health, especially if your arthritis is advanced.