Labor pains. Most women know they are about to deliver when they begin labor. Some women start labor but do not realize what is happening. Another clue is your due date. It is normal to begin labor anytime after 37 weeks gestation and before 42 weeks. First time mothers have never experienced labor before and sometimes you can read every book in the world and still not know. My wife was one of them. :).
Ready to deliver? You'll know that your ready to deliver when you go into true labor which is defined as progression of events that eventually lead to childbirth. At times, it unclear whether a woman is in labor or not, and can only be decided after progression of the process.