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If this person has a problem with substance abuse, gambling, binge eating, or other destructive behaviors, there's a good chance they're a pathological liar.  For example, you may notice a co-worker drinking extra at a company party. You may see them filling up their drink when no one else is at the bar, or even notice them carrying a flask. You may also notice you don't see one co-worker eat at lunch, but sometimes find evidence of food having been in their office. They may be very secretive about eating habits, and habitually turn down offers to get food with co-workers. Pathological liars are often disconnected with reality. A lot of times, they may believe parts of their lies themselves. They may be deluded about themselves and their abilities.  A pathological liar may have a tendency to exaggerate their importance. They may see something innocuous, like a compliment from a boss, as a sign of personal greatness. When recounting the compliment, they may fluff its importance. A pathological liar may lack basic life skills, but  may not see this as a problem. If the person has a distorted view of reality, they may honestly believe what they are saying. While this is not true of all pathological liars, consider the possibility that the person is not lying out of malice. Pathological liars tend to have unstable relationships. Consider anything you know about this person's relationship history. Look for any warning signs of instability.  Does this person have stable friendships or romantic relationships? A lack of longterm friends, and a series of failed romances, may indicate a pathological liar. A pathological liar may also be estranged from their family. A pathological liar tends to bluff their way into jobs. A pathological liar may have many jobs on their resume. However, most of these jobs will be short term. They may also dodge questions about why certain jobs did not work out longterm.  For example, a pathological liar may have a long resume. Most of the jobs will only have been short term. If you ask the pathological liar about their career, they may dodge questions. In some cases, a pathological liar may have moved around a lot due to sudden career changes. Pathological liars often burn bridges with employers.
Look for underlying secret habits. Consider whether the person lives in reality. Think about the person's relationships with others. Study the person's career.