Excellent question.
As it's currently the tail end of grape season, we get a ton of winemakers and wine enthusiasts coming into our shop, and I get to talk to quite a few people that I would consider wine snobs.
You're a snob if:
1. You believe that your opinions about the taste and quality of wine are fact. (They aren't. They're your perceptions, and the opinions of others are just as valid.)
2. You consider those who don't share in your opinions as beneath you or unworthy of your respect. (People perceive different tastes in different ways and have many levels of sensitivity to certain flavors. Some people just aren't going to like the same things that you are...it certainly doesn't make them unworthy of respect. Tastes are completely subjective.)
3. You automatically dismiss wines based on price, region, etc. (There is a lot of art to winemaking. I've had completely undrinkable wines that were sinfully expensive, and great wines that I paid under $10 for.)
4. You honestly feel that you know so much about wine that trying to learn more is a waste of your time. (A friend of mine has been making wine for 25 years. He recently graduated from UUC Davis with a degree in wine. His first comments to me about it were 'I thought I knew something about wine. I knew probably about 5% of what I thought.' An enthusiast knows that there is always something about the subject that he/she has yet to learn. A snob is too busy telling others their about vast knowledge to bother trying to find that 'little bit' they don't know...besides, if it were really something important they would have discovered it long ago.
Generally, I think if you have the presence of mind to ask yourself 'What's really a snob, and am I one of them?' then you probably aren't.