Definition of liabilities
In poker, when we talk about a passive player, we are talking about a player who usually does actions rather than aggressive actions. Pass, limper, fold and call are actions passives. Bet, raise, 3bet and 4bet are aggressive actions. A passive player is a player who will mostly do passive actions. An aggressive player might for example check-raise a flop draw. A passive player will check-call his draw. The only time the passive player will raise is when he has a nuts.
How to identify the passive player?
If you have a tracker like Poker tracker 4 or Xeester, you will be able to display a statistic called "Aggression Factor" in your HUD. This is usually a good indicator of a player's aggression level. The statistic will make a ratio between the player's passive and aggressive play and will indicate an aggression rating. To identify aggressive gamblers, be aware that 2.0 of "aggression factor" is the degree 0 of aggression. At 2.0 one is "normal" aggressive. At more than 2, one is considered aggressive and at less than 2, one is considered passive. You will sometimes see gamblers with aggression indicators of 0.7. Against such passive gamblers, give full credit to their aggressive gambling!