I think this is a situation where a more subtle approach would work better than a direct question.
The next time you guys are sitting around relaxing, offer her a foot massage. Or start with the shoulders if that feels more natural, then transition to the feet. Give her a good one - keep at in until she's putty in your hands. And when she's totally relaxed and into you... as *gently* as possible, run a finger up one of her soles. Her response will tell you a lot. If she hates being tickled, even here when you've connected it to a wonderful massage, you might want to find yourself another target. If she giggles and doesn't pull away, or if she does pull away, but gives her foot back to let you keep massaging, then you have your opening. Keep on massaging and tickling, or go for a more intense bout of gentle, fun tickling, depending on her response and your style.
Good luck!