In order to find your enemy’s weakness, you must learn two things: perceptiveness and patience. Watch your quarry and learn all about them. Even their smallest quirks might prove useful. How they talk to their friends, how they treat their servants, how they behave on the Holy Day. You never know when you'll find something of use.
And patience is important because something might prove to be just a perceived weakness. Remember, your enemies are smart, too. They didn’t get to where they are by being ignorant of their own vices. Sometimes they leave something out in the open and use it as bait for those who lack patience.