I think this is a pretty good article about how to approach the subject with a vanilla partner: Initiating a Discussion About BDSM Interest with a Vanilla Partner
There have been many threads about a similar situation here, so you might want to browse back a few pages and check the BDSM Library (there's a sticky on top of this forum for it), too.
Then again, I think just asking for a spanking or for him to take you rougher or something like that during sex is a pretty easy and innocent enough way to see if he's into it. Later talk about how great it was and how you've been thinking about it for a long time and what more you'd like to experience etc.
If he's clearly not into it when you ask for a spanking, you can always say afterwards that you got carried away with your words and requests because the sex was so awesomely hot. And even if your suggestion doesn't amount to anything when you're asking, at least you've said something and it'll probably be easier to get back to the subject later on.