2. Your in the mall looking at clothes and find yourself saying "no, I can't wear that, that's something Penny or Sheldon would wear."
3. You find yourself surfing the internet for articles on string theory and other theories mentioned in the show.
4. Any tall lanky guy wearing a long sleeved t-shirt under a vintage t-shirt reminds you of Sheldon.
5. Any guy who hits on you in the bar you think is a whole lot better than Wolowitz.
6. You place an ad on Craigslist looking for your own Sheldon.