I'm not sure why he asked about other joints, so would only be guessing. My knee doctor (not a surgeon) diagnosed my arthritic knees using X-rays, history (one knee first developed chronic pain after an injury that led to cartilage erosion), pain levels, and what types of activities caused the pain. As diagnoses go, arthritis is fairly straightforward. X-rays were what showed the knees to be bone-on-bone, which is why I was handed off to a surgeon, because there was nothing else my knee guy could do for them.
As we age, most people get arthritis in the joints. I too have some in my back, though not bad or causing any problems. X-rays showed that, too.
Next time you see the surgeon, ask him why he asked about arthritis in other joints. Maybe he was wondering if you had arthritic hips which might act up with knee surgery, or bad ankles. I think his answer would be interesting.