What could you do if you were caught green-handed?
There are dozens of techniques to move an object around the hand and vanish it. There's the 'French Drop.' It allows you to fake take something and retain it in the hand still. For a long, skinny object like a joint, you can do this easily.
If we are addressing a bag of weed, specifically, it could be handled in various ways using [an approach to communication] called NLP—Neuro-Linguistic Programing—meaning a skilled magician can apply the best strategy to either charm, entertain, influence, or manipulate the situation.
Every case is different, but let’s say I was caught with some weed. Besides that obvious approach of vanishing it with sleight of hand and fast thinking, I might say the opposite of what a cop would expect like "YES! This is weed! My friend James let me borrow it so I could film this funny YouTube skit. James has a card and legally uses weed… I do not use it!"
Having a YouTube video script always in the pocket to confirm the “story” could be beneficial as a way to plan ahead. However, you’d be using that as a contingency in the event you did not come up with a strategy on the fly using observation, magic skill and/or influence.