Piezo impact sensor will be your best bet.
You will detect more impacts (like sticks hitting the ground) so you may set a threshold: above a certain value it's a ball strike, below a certain value it's something else.
That of course goes wrong when moving a ball gently forward, as you only detect the hard hits, which in turn could also be you hitting someone elses's stick when fighting over the ball.
Now if you could place a strong magnet inside the ball, you may be able to detect that magnet using a hall effect sensor (range may become an issue for this). Then you have the hall effect to detect whether the ball is very close to the stick, and the piezo to record the moment of actual impact. That should cut down seriously on false positives and false negatives.