How do you find your opponent's weakness?

[Chili Palmer]

Bet more and watch if their arm shakes

I had a team member in leagues who would try to get in a better player's head with (seemingly) earnest questions.
"I can't get any rhythm in my stroke. Do you exhale as you stroke the ball, or hold it until after?"
"How do you shoot so straight with your elbow in that position?"
"I gotta try that stance. Maybe it'll work for me, too."

Or, "How is it you can consistently do 3 backstrokes then shoot"...a friend of mine told me that one.

 

[HawaiianEye]

This is one of the reasons old-school gamblers preferred two-shot rollout or pushout 9-ball to the one-foul BIH crap that is played now.

After a few games you could get an idea of which shots they would take or pass back after rolling out to certain places or shots on the table.