You know you're an introvert when you

Are extremely happy when you get to be alone
Can have entire conversations with yourself
Create complex scenarios based on what you will do next.
Practice the lines you are going to say to someone over and over, and still fuck it up.
Know what you want, know that the other person doesn't know what you want, and still pretend like the other person knows it.
Have few friends, but those friends are ridiculously close to you.
Have friends that 95 percent of the time, were the ones to start the conversation.
Aren't necessarily shy, but that's what everyone says, and you just roll with it.
Don't agree with a point, but don't have enough time to practice your points, so you just agree anyway.
Get extremely tired after a party(if you're even invited, that is).
Don't want to go to a party, but sometimes still want to get invited anyway.
Want a choice to say no when you ARE invited to a party.
Are known for being the quiet guy/girl, or you are not even noticed at all.
How do you know when someone is an introvert? Talk to the person and see their reaction. If they are surprised that a human decided to talk to them, they are probably not used to it and so, are introverts.