Depends if you are Billy no mates ,never smoked,gone pub etc.
Sure I have had odd smells from cars but couldn't say for certain what it was
You'll smell it just cycling or walking. You'll know. If someone is holding a wrinkled looking ciggy, and it smells weird and strong, that's weed. Think of very pongy boiled cabbage on steroids.
It's not something that was that obviously smoked in pubs (when smoking was allowed) - your usual high street is where you'll smell it.
It really is surprising how much is smoked by people commuting in cars. All sorts of cars, from builder's vans, executive cars, nobbers in modified cars, and your family run MPV or SUV- obviously commuting at silly hours you'll not see many cars. Monday mornings and Friday afternoons were the main days.
My current commute is off road, so it's usually someone walking that's smoking it, or a haze drifting out of houses that back onto the Fallowfield Loop.