once you smell MJ, you will never forget it.

I first smelled it as a little kid (age 4-5) in a cousin's room whom I of course didn't know smoked or what pot even was. Years years later when I first got a whiff of the stuff, I instantly remembered the smell. So I wouldn't bet that they don't remember.

If you asked them jokingly if they remembered ("you're sooo old dad, i bet you don't even remember what weed smells like even though you used to be a stoner") and they say they don't, they're bullshitting you so you'll slip up.