Well if she learns the fertility awareness method http://www.fertilityuk.org/ which involves observing certain indicators, charting them and learning when you are fertile then she'll know!
A woman is fertile from puberty to menopause, it's only the degree of fertility that varies, depending on a variety of factors, most commonly affected by the menstrual cycle.
I've noticed a bunch of women are most fertile for the year after quitting birth control pills, and the year after having a child (unless she is breastfeeding). Of course, these are just my observations. Nothing scientific.