(This is today's bit of advice from the book Safety Toolbox.)
When you have no previous experience playing against your opponent, observe his game closely. Start by looking for perfection - things that he does well. This includes stroke, accuracy, cue ball control, speed control, stance, etc. Then look in the same areas for things that are imperfect.
Closely watch his stroke and ball control. Is it precise and crisp, or careless and haphazard? If he talks with others, listen to his comments and his tone of voice. These provide valuable insights to his mental state of mind. If you have the opportunity, watch him play others. This could help identify shots that give him problems. If he just bangs away without thought or if he takes his time - he has revealed his approach to problem shots.
Pay attention on how precisely he controls the cue tip/cue ball contact point. Does he have an unconscious bias to hit off center (left or right side)? This information can tell you how to manage his shooting opportunities.
Observe how he uses side spin (English). Is he consistently successful or are there failures? How does he react to failed attempts? Does he moan and groan or appear to be working out what went wrong?
In addition to identifying general and specific capabilities, set up several different safety types and temptation shots. Observe his response. Does he make smart decisions, or does he attempt some fantasy shot? The answers provide insight into his thinking and playing style. All of these little details are assembled into a workable playing profile that helps you design strategy and tactics.