Not that anyone doubts Sheldon Cooper's geek status by now, but here's a couple more telltale signs he's a geek:

1. He uses behavioral psychology principles to modify the behavior of people in his life. Doing so, by itself, isn't geeky. Every dog trainer does it. What's geeky is citing B.F. Skinner as a source while doing so.

2. He has just a little bit of trouble relating to normal human beings.