At decent levels of play, serves are directed towards the backhand side more than the forehand. So where do you think the next serve is going?
Also, most players consider a switch from forehand to backhand the most accurate and easiest to perform. So which grip should we start with?
First serve usually bounce lower to you than kicking second serves.
Slice serves tend to skid and bounce low.
Tendencies. That's all you have to go by.