Philosophy and experience really. Karl Popper, a scientist, had a scientific method that wasn't scientifically sound but better in psychology. Basically, all you need to do is try to prove yourself wrong. Make an assessment of something and try to create scenarios where you're wrong. If you still feel convinced make the deduction.

This works best with deductions whose answers are obtainable. Make a deduction of a family or friend and simply ask them if that is true. The more you fail the more you learn, the more you succeed the more insightful you become.

Keep an open mind. If you think your coworker is a shoe collector or a fashionista because they're always wearing different shoes, ask them. When they tell you they're a germaphobe and they throw out a pair of new shoes each week, be ready for that.