This is much more general than just D/s, but it has some great thoughts: http://www.xeromag.com/fvbdsm.html
If you think she's submissive but aren't positive, is there a reason you don't just ask? Do you think you might be interested in kink yourself?
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.