Narcissists Tell on Themselves
Pathologically narcissistic people are known for being duplicitous, or deceitful. They have a reputation for being fake or for hiding who they really are. This reputation is deserved. However, the majority of them are not very good at it. Little pieces and flashes of what they are hiding do show through, and people do see these flashes. It's just that it's often something that is so opposite of what the narcissist is claiming to be that people will ignore or rationalize it away, believing it to be out of character for the narcissist and convincing themselves that they are over-reacting. It is only over time that people realize this angry, demanding, needy or unpleasant behavior is not out of character for this person at all, and in fact it is their primary way of being.
It is true that narcissistic people make a very deliberate attempt to present themselves to others a certain way, and this is often not a reflection of how they really are at all. But most people do notice certain things that don't add up or don't jibe with the rest of the picture. For instance, someone may boast on a date how wonderfully they treated all of their exes but they may also use derogatory language about these people or make inappropriate or off-color remarks about them throughout the night, things that don't really go with the "Mr./Mrs. Wonderful" picture of themselves that they're trying to paint. This is one of the ways they tell on themselves, by saying and/or doing contradictory things. This should not be ignored, because if someone is doing this, it means at least one of the things they are saying is not true and may be an indication that the person is hiding something or has some type of perception problem.
Another way pathologically narcissistic people may tell on themselves is by blurting things out that have nothing to do with anything. They simply blurt things out. It usually indicates something they've done wrong, or feelings they have for themselves. For example, an innocent question may provoke a very hostile reaction, with the narcissist saying, "I'm not lying!" even though they've been accused of nothing at all. Of course, this makes many people suspicious and it just turns into a bigger and bigger problem.
A third way that a pathologically narcissistic person can tell on themselves is with projection. That is to say, they will accuse other people of what they are doing or what they are feeling. Projection is generally a core feature of any type of narcissism. To the narcissist, their feelings are facts. If they feel that you are doing or feeling or thinking something, that's a fact. When they are confronted by things they find uncomfortable or hard to deal with about themselves, such as feelings about themselves that are painful or things they know they should not have done, they often deal with these things by projecting them onto other people: I didn't do that. You did that! I don't feel that way. You feel that way. In this way, they avoid the feelings that accompany these things and deny their existence. Denial is the narcissist's armor, and all of this is part and parcel of their overall denial problem.
In the end, no matter how good narcissistic people may be at hiding who they are, something always shows through. Make sure you trust your instincts here. If something doesn't add up, don't ignore it. What doesn't seem like a big deal now could be a big deal later. That doesn't mean you have to make a big deal out of it. You may not choose to call the narcissist on it, and you don't have to. It may be better if you don't, in some cases. But make note of it. Don't just rationalize it or push it away. The last thing the narcissist in your life needs is a partner in their denial.