Cervical fluid is produced at the cervix, throughout the entire cycle. Examination of this fluid can fairly well tell you what point you are in your cycle. The best way to examine the cervical fluid is by swiping toilet paper or your fingers across the opening of your vagina. Or you can insert a finger or two and draw it from the opening of the cervix itself. If you choose the latter, be sure to wash your hands thoroughly beforehand, and make sure to have your nails trimmed.
Long nails can nick the cervix and cause it to bleed.
You then take the fluid and put it between your finger and thumb and check to see what type of consistency it is. During the first part of the cycle, the uterine lining is being shed. This can mask the cervical fluid making it tough to determine if the fluid is actually fertile or not. And yes, a woman can be fertile during her menstruation, though it is rare. I personally know a woman who conceived the 8th day of her cycle.