Most Helpful Guy
If i were you i would not be subtle but don't say a word. Girls don't like to admit they have desires ESPECIALLY if the guy might think its weird or laugh at her etc. Girls are social and emotional, there not going to handle that well. there's a part in the movie called the secretary where she is hanging out with i think his name was peter, she just says fuck it, and places herself across his lap and basically says don't be scared.
Well ole' peter wasn't up to the task so needless to say it didn't work out between the two. If its the kind of guy you want, then force him to make a choice, its really simple actually, next time he is sitting in a chair, just get over his lap, doesn't get any more obvious then that. Sounds difficult i know, but at least you will find out if he is up to the task, truth be told- im a guy that does it to girls all the time, these other guys can pull the "just be a naughty girl" routine all they want, but i can promise you not every girl is in it for "arousal" not every girl is in it to be in a dom/sub relationship, and not every girl on the planet is the same, some literally want it old school, they screw up, they see getting spanked as a way too "correct it" etc. there ALL different.
Find out why you want him to do it, then force him into a position to be able to do it. If he doesn't (i mean ya he will be a little freaked out, thats normal the first time) if he decides to do it, he will start out "playfully" (you should already know in your mind how "far" he can take it and be prepared to defend it if he tries to go full dexter)
Most Helpful Girl
Wait until you're in bed and on all fours, then wiggle your bum at him extremely provocatively until he gets the hint, if he doesn't then I suppose you'll just have to tell him... with words... :/
If you say it in the heat of the moment it may feel less awkward...