Only Adam Lambert really knows for sure if that lit cigar he passed Miley Cyrus at his Halloween party contained the devil's weed ... and over the weekend, he dropped a pretty big clue.

Lambert was at LAX when a photog asked point-blank about the photo TMZ posted, showing the former "Idol" superstar handing Miley something that looks like (and probably was) a big, fat blunt.

You gotta see Lambert masterfully bob, weave and deflect when asked about it -- but then came the big moment ... when the photog said, "If it was a joint, say nothing."

The reaction pretty much says it all.

Now excuse us ... we have some Funyuns to eat.