You can know if someone is an introvert if they keep loving to stay indoors for no good 

reason,many introverts also love to be alone so whenever you see someone that loves to be alone 

then that could be a sign that he or she is an introvert,many introverts are nerds,so whenever 

you see that nerd in your class there is high probability that he or she is an introvert,some 

introverts love to stay at home and browse the internet or read alot of novels,when you see 

someone who is so addicted to the internet,the person could be an introvert...

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.