How can you tell the difference between fertile cervical fluid and regular discharge?

I don't understand how you can tell whether you're having fertile cervical fluid or just everyday discharge. Can someone help me? I don't quite understand how this works. I just recently found out about fertility charts and how you chart them, but I come from a religious state, and they don't really teach you anything about sex in school. Everything I learn has come from what others say or from experience.