Probably lots of ways to get clues... but seriously... at this point in your life, why not just ask?

Barring that...

You could bend yourself over his knee and see what he does.
You could grab the headboard and "struggle" and see what he does.
You could get on your knees and tell him all he has to do is ask and you'll obey... and see what he does.

Less overt, but no more subtle, instead of a selection of coffee table books... set out a Playboy, a Penthouse, and a couple of Robert's or Ferris graphic novels and see which ones make him squirm the most.