You know you're high when you start talking about fundamental concepts in a completely different light to the point where it actually creates an intelligent discussion between yourself and a sober person.

You know you're high when you finish and you're the only one laughing, and you were also the only one laughing the entire time you were telling that story. Your friends might even be high but they just sit back and don't react to your story at all. Then you start to wonder if they think you're annoying or crazy or retarded. Then you worry they won't let you smoke with them again. Then you mention your theory that they think you're crazy, and they let you know it's cool, they were just thinking. And this all stays on your mind enough to be worth spending an hour thinking about, and a paragraph typing about. Yep, you're high.

You know you're high when David Letterman is funny.