How do you determine if a horse has a smooth walk?

Get on in shorts bareback. If your crotch hurts when you get off, or if you can't carry a water bottle with you because it sloshes all over, then no, your horse does not have a smooth walk.

To be more scientific about this, the slope of the shoulder blade can tell you. The steeper the slope, the less smooth it will be. Also take a look at the fetlock, the thing below the pastern and above the hoof. If those are almost straight up, it will be uncomfortable, and whereas more flat will be smoother.