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...you have to act normal. After reviewing all the different cues as to how you can tell if you're high, I realized the biggest sign to knowing if you're high is that you have to act normal. Let me tell you though, that is probably the hardest thing to do and that's something they don't teach you in school or anywhere else for that matter. It might be the hardest thing to do, but it's also the most fun. Next time you smoke, think about it. You're gonna get to a point when you have to act normal. When this point comes, that's when you know you're officially fried. That's how I know if I'm high. When I have to act like I'm sober...I know I've reached a good plateau for myself.
Take an acting class, seriously. I took acting classes for 8 years throughout middle/highschool and college. They have so many benefits, from getting people over their fear of speaking in front of large groups (just about everybody has that fear, don't deny it), to learning how to bluff your way out of most situations. Acting is, at its core, impersonation. You have to truly become the character on stage, you can't just be representing them. Thus, the more acting experience you get, the easier it is to assume and change personas at will. Now, I'm not talking about switching, for example, from a neurotic accountant to some millionaire playboy at the drop of a hat, but being able to, say, when your high, ACT as if you are sober. With practice, you can easily learn to act your way out of a lot of bad situations, and keep your high going.
They also (inadvertantly) teach you how to become an good liar.