Don't ask a user. Even if they said they're "intelligent" that still doesn't disprove that if they didn't some weed, they'd be even MORE intelligent.
So the answer you get is inconclusive.
Same if they're total morons or stupid. Maybe they were that way before they started smoking weed. Plenty of female airheads i know who don't smoke weed, but stupid as a doormat.
The only way is to study scientific studies. Believe a researcher over than Raggae Rob down the hall.