Almost every player has a shot that can be exploited.
Initially, most players' forehands are stronger than their backhands. If you are not sure which side is weaker, try attacking their backhand first and see what the response is. It may be necessary to hit the ball to their stronger side initially to open up the weaker side since some players are very good at covering up their weaknesses.
Often, a player's second serve can be a weakness. Look to attach their second serve by standing in and getting the ball back to a spot that makes them move. A good baseliner is often weak at the net and has trouble with the short balls, moving forward and volleying.
The reverse can be said about aggressive net players--it is difficult to keep them back but try getting the ball deep in the court, using lobs if they have already come in, or getting them on the run at the baseline. Some players are much better at returning second serves and struggle on the first serve regardless of how you hit it-get a lot of first serves in to exploit this weakness.
Try to use the warm up to help locate your opponent's weaknesses but remember things may be different in an actual point so be ready to adjust accordingly.
Steve Annacone, USPTA Elite Pro, is the Director of Annacone Tennis (http://www.annaconetennis.com) and MyHamptonsPro (http://www.myhamptonspro.com). For details on lessons and clinics throughout the Hamptons contact firstname.lastname@example.org or email@example.com or call 865-300-7323