The Billings method uses the cervical fluid to help determine when you are going to ovulate. I prefer to use a method like Fertility Awareness Method that combines cervical fluid, basal body temperature and cervical checks. The problem with the billings method for myself is that I don't produce good quality cervical fluid so relying on that alone wouldn't work for me. But combined with everything else I know my body really well and am able to tell when I should ovulate and then confirm that it did happen by the rise in my temp.
Read the book "taking charge of your fertility" by Toni Weschler and you will get an idea of what I'm talking about. You can use the free website www.fertilityfriend.com to track your fertility and it will also help you to interpret your signs.