The truly great poker players are great because they prey on their opponent’s weaknesses. 

Finding and exploiting weakness should be the number one goal of every poker player, but before you can identify a weakness, you need to have a good understanding of what to look for. 

Defending the Blinds: If you notice a player defending his blinds too liberally, then you can safely ascertain that they do not understand the importance of position. 

Any player who does not understand the importance of position is lacking a fundamental principle of poker. These players should be attacked in Poker Sit and Go Tournaments whenever the opportunity presents itself.

Calling Three-Bets Out of Position: Another positional mistake is calling too many three-bets from out of position. The aggressive nature of the modern game makes it is just too difficult to play in three-bet pots out of position. 

There are just too many times when you will be forced to fold on either the flop or turn to make this play profitable, and when faced with this type of player that is what you must do; pile pressure on the flop and turn.

High Open Raising Frequency: The weakness here comes in the hand range that this type of opponent will be playing. It will easy to three-bet in position and then pound the flop and turn with impunity.

Bet Sizing: There are a tranche of players who give away the strength of their hand by the size of their bets. These types of players bet small when weak and large when strong and are very easy to manipulate once you spot this tell.