I've narrowed down the play in my left front wheel to the upper control arm. When I shake the wheel left to right I can see the upper control arm move, but I can't tell if it's because of the upper control arm bushing, or the upper ball joint where the control arm mounts. How can I narrow it down?

The issue seemed to come suddenly. I could hear a bit of a knocking from the front end when I was braking going downhill. By coasting downhill with the engine off, I was able to eliminate other suspects. It seems to give a knock as I come to a stop, and then another knock as I move again.

Thanks,
Ed