Being rather new to this myself I don't want to come across as a know-it-all or anything, but reading your post gave me a few ideas you might try to gauge the situation. Bear in mind, my wife and I have the oposite roles, but part of us finding out what tickled our fancies (pun intended) I thought quite a bit about this for my own part.
Watching my wifes progression from a simple pair of furry handcuffs through to blindfolds/wristcuffs chained to the ceiling and caning, I've identified a few key reactions as to what works and not. I would imagine a simple test to see if he is into the submissive role could be something like this: tie him up and blindfold him, stripping him of some control. Tease him in whatever way you normally do when having sex, licking/tickling/biting or similar. If the whole powerlesnes of the situation increases his arousal, I would say you are on the right track.
Reading this through, I can see how it might look a bit näive to simplify the submissive role to a simple reaction, but I still think this could serve as an indication at least. Hope this helps, if not just to provoke an answer from someone a bit more knowledgable...