I've heard a lot of pointless arguments about "Nerd Cred" online and at comic conventions. With a lot of nerd culture going mainstream over the last decade or so, there is concern about authenticity - who is a "real" nerd and who is just a poser, especially so with the "gamer girl" trend of the last few years.
It's not a terribly interesting or important debate. However as a former English teacher, I believe strongly that words matter. As much as people claim to hate labels, we need them to accurately describe and characterize the world around us. The word "nerd" ought to mean something.
My criteria for thinking of someone as being a nerd is the following three characteristics:
1. Above average intelligence
2. An unconscious indifference to trends and social dictates
3. An obsessive passion that overrides all other interests, including relationships with human beings
Nerds are not dumb. Furthermore they are not trendy. A "trendy nerd" is a hipster whose priority is being seen as a nerd, not actually pursuing knowledge or some interest about which they are passionate. Real nerds don't actually worry about being acknowledged as nerds. They are sort of like gangstas in that way. Finally, nerds are obsessive about something to a degree that impacts their social lives.
This isn't to say nerds have no friends; quite the contrary, many have several friends from D&D campaigns, online gaming, and fantasy meetups. The difference is the order of priority. For non-nerds, hobbies are a way to spend time with friends. For nerds, friends are a way to enjoy a hobby.
Here's an easy test:
What sounds like a more fun Friday night:
A. Some version of going out with friends, drinking, partying, exploring a new club, socializing, etc.
B. Home alone with your computer, your games, your books, and time to hack away on some side project.
Think about how you spend your free time and your weekend nights. If your honest answer is B, you meet the third condition.
A lot of people satisfy one or two of the above conditions. I know some very smart people who are passionate about their work, yet obsessive about following trends and social norms. I know smart quirky people who are not constrained by society but they have no strong interests or passions. I've met obsessive and weird people who just lack the general intelligence to be called nerds. I think of people in that last category as 'geeks'.
Regardless, the above is just my own personal criteria and it works well as it properly excludes 99% of humanity which is roughly the percentage of humanity to which I cannot relate.