Pay Attention to Gestures and Unconscious Signals
Just as Sigmund Freud said, No one can keep a secret; if they don’t speak it through words they will speak it through their body language. If you can’t figure out someone’s weakness through simple conversation then maybe you should watch what they do. It is not what you look for but where and how you look. If you have a feeling someone has a weak spot but can’t figure it out through conversation, you should continuously bring up this issue somehow and watch their body language and facial expressions, that should give you more clues if this is their weakness or not.

Find the Helpless Child
Most weaknesses begin in childhood, before the self builds up defenses to compensate for said weakness. Maybe as a child you were spoiled in a certain area, or maybe an emotional need was left unfilled; as you grow older, the too much or lack of may be hidden but never goes away. It only takes someone to expose it and they can control you through it.
Look for Differences
A strong trait commonly hides a weak one. Those who think they are big and bad are usually cowards. A shy outside may often hide a tough soul. The uptight are often screaming for some excitement. The alone are often screaming for attention. By scoping beyond appearances, you will commonly find their weak trait that hides behind the strong one that coincides.

Find the Whole in their Armor
Sometimes when looking for someone’s weakness, it is not what but who that matters. Usually there is someone who has a great deal of power, or a tremendous influence over the person on top. For people like this, that person may be their weakness. You could insult them all you want with no affect but, as soon as you insult or speak up on this person you could affect them deeply.
Fill the Void (Hole)
The two main emotional voids are a lack of self-worth or insecurity and unhappiness. The insecure are suckers for any kind of social acceptance, as for the unhappy, looks for the reasons why they are so unhappy. The insecure and unhappy is the people that are hardest to