Narcissists tend to feel entitled, with (over)inflated views of themselves and low levels of empathy.
The "traditional" way to formally determine if someone had a narcissistic personality was to administer a 40-question survey known as the Narcissistic Personality Inventory. (You can try it for yourself here .) The inventory measures overall narcissism and its component traits, but takes a long time to complete (as you will find if and when you try it yourself). That can be problematic for researchers, especially when they are only interested in an overall gauge of someone's narcissism. Part of the challenge is that subjects get bored and distracted, and then drop out of studies.
But researchers have recently found that, if you just want an overall view of narcissism, you can seemingly replace that 40-question inventory with a one-question assessment: "How narcissistic are you?"
The key insight behind this approach is that people who are narcissists aren't embarrassed by it, so they don't hold back on saying so. People who are not narcissistic would feel ashamed to be seen as such, and so they tend to rate themselves low on the scale.
"People who are narcissists are almost proud of the fact," says Brad Bushman, one of the study's co-authors. "You can ask them directly because they don't see narcissism as a negative quality—they believe they are superior to other people and are fine with saying that publicly." 
The development of this Single Item Narcissism Scale was not based on just one study: The researchers conducted 11 different studies with over 2,200 subjects to come to their conclusions. You can read the full study at PLOS ONE .
Why is this of particular interest to people like me, who study social media? We want to learn to determine whether someone is a narcissist by analyzing their social-media profiles—essentially, we want to do it without asking any questions. To develop computer models to aid us, we need a lot of "ground truth" data—that is, data points that tell us both what is in people's online profiles and what their actual narcissism score is.
To get that, we need volunteers to take a narcissism survey.
But while it's difficult to get people to take a long survey, it's fairly easy to get them to answer a single question. My colleagues and I are running a study now to determine whether we can analyze the text of people's Twitter posts and use that to determine if they are narcissists. This single narcissism question has made our job so much easier; something that would previously have taken 15 minutes now takes 30 seconds.
That means more data and better results.
Want to be one of our guinea pigs? You can participate right here.
 "Just one simple question can identify narcissistic people" Science Daily. August 5, 2014. http://www.sciencedaily.com/releases/2014/08/140805150645.htm
 Konrath, Sara, Brian P. Meier, and Brad J. Bushman. "Development and Validation of the Single Item Narcissism Scale (SINS)." PloS one 9.8 (2014): e103469.