Is Shaggy a stoner? That is the topic of Season 1, Episode 3 of Harvey Birdman: Attorney at Law.
From Trakt.tv: "Shaggy & Scooby, while en route to solve another mystery, are pulled over by a State Trooper. Their natural gigglish and dopey behavior leads him to believe they are under the influence of illegal substances, and they are taken to jail. Harvey represents Shaggy & Scooby, while Spyro is the prosecutor."1
The episode begins with Shaggy and Scooby being arrested and put in jail.
The episode ends without a verdict being given. The good news is that Scrappy-Doo gets his comeuppance.
At the end of each Scooby episode, Shaggy is with the rest of the Gang as they summon a local law officer to arrest the villain(s).
Law officers have a tendency to detect the scent of marijuana on a person who is using the stuff.
Surely, Shaggy wouldn't wait around for the Law to arrive if he were smoking pot.
Yeah, yeah. You don't want to be called Shirley. That's beside the point.
Information Source:
11x03 Shaggy Busted. Trakt.tv. Retrieved from https://trakt.tv/shows/harvey-birdman-attorney-at-law/seasons/1/episodes/3