1. Height of ball bounce on first and second serves.....you now know how high to hold the racket... Fast serves, you shorten your backswing.
2. Tendency to hit spin serve as first serves, and when....
3. Forehand...how consistent, how strong, can it handle low and high balls?
4. Backhand..can he keep it low, how are the lobs, can he hit offensive shots with it?
5. Does he come to net? Does he have an overhead? Does he tend to volley short angles or deep placements?
6. Return of serves. How does he handle high backhands? Serves into the body? Wide forehands?
7. How does he move after 4 shots?
That's a LOT of info for you to process...