Well that’s tricky isn’t it. I think one of the best ways might be to ask about the anklet. You might say that you noticed it or liked it or that you don’t see many women wearing them. That might get the ball rolling.
Outside of that you might have to allow her to ‘catch’ you looking at that type of porn or ask her advice about it, about a friend who is thinking about entering the ‘lifestyle’ and wanting a woman’s point of view. Just any way to see if she’s willing to talk sexually and then take it from there.
I love the suggestion of simply and innocently asking about (and complimenting) the anklet. My wife wears one, and that’s exactly the way we’d prefer her to be approached.
The porn thing is a little more problematic … good way to get fired if you’re guessing wrong.