You should know the racket used by your opponent, including the straight version or the horizontal version, and then know whether it is reverse glue or granular glue (raw glue, long glue, positive glue), whether it is domestic or imported.
Spin. When the opponent serves, it's time to pay attention to it. There are many ways to observe, including indirect observation and direct observation. Direct observation is the change of the position and hand shape of the opponent when he touches the racket in an instant, the movement of the body with the hand shape, and the force of the wrist and fingers. The indirect judgment is the ball speed.