#Passive checks are different from active ones#

A passive check is a special kind of ability check that doesn’t involve any die rolls. Such a check can represent the average result for a task done repeatedly, such as searching for secret doors over and over again, or can be used when the DM wants to secretly determine whether the characters succeed at something without rolling dice, such as noticing a hidden monster. (PHB 175)

As per the rule on Passive checks, if the enemy is using their passive Deception it's likely you would use your passive Insight.

The idea behind the ability is not to modify how well you passively catch liars but how well you actively catch them. Generally when someone lies to you they make a deception check vs your insight. In this opposed roll situation you have a 40% chance of a better or equal result (if you have the same bonus to deception/insight).

###Usefulness will depend on your DM### As with a lot of non-combat abilities in D&D their usefulness is determined by how often your DM calls for them. If you DM only ever uses passive checks for lying/insight then this ability is generally worthless. If you find your DM doing this let them know that it feels like their making a class feature of yours feel like it was a waste.

Passive checks are different from active ones

A passive check is a special kind of ability check that doesn’t involve any die rolls. Such a check can represent the average result for a task done repeatedly, such as searching for secret doors over and over again, or can be used when the DM wants to secretly determine whether the characters succeed at something without rolling dice, such as noticing a hidden monster. (PHB 175)

As per the rule on Passive checks, if the enemy is using their passive Deception it's likely you would use your passive Insight.

The idea behind the ability is not to modify how well you passively catch liars but how well you actively catch them. Generally when someone lies to you they make a deception check vs your insight. In this opposed roll situation you have a 40% chance of a better or equal result (if you have the same bonus to deception/insight).

Usefulness will depend on your DM

As with a lot of non-combat abilities in D&D their usefulness is determined by how often your DM calls for them. If you DM only ever uses passive checks for lying/insight then this ability is generally worthless. If you find your DM doing this let them know that it feels like their making a class feature of yours feel like it was a waste.