You can figure it out without temps, you just need some information about how to make and interpret your cervical mucus observations.
Taking Charge of Your Fertility is a great jumping off point to get the basics, but not quite comprehensive enough for mucus only charting. For that, you may want to read "
The Billings Method: Using the Body's Natural Signal of Fertility to Achieve or Avoid Pregnancy" by Dr. Lyn Billings and Ann Westmore. Or you can also learn the Creighton Model, a medical form of classifying mucus - but you need a teacher for that one.
www.fertilitycare.org
Good luck!