Most first babies are late. You won't know before your labor when it will start. It is one of the great mysteries of life. If someone figured out how to know when babies are going to make their appearance, they'd be rich!

Your due date is called an Estimated Due Date or EDD for a reason. They can only estimate when s/he will get here.

Trust me, when it is real labor, you are going to know its real labor.

I second Mabel's response. You'll know when you're in labor. The rule of thumb is call the hospital if your contractions are 2 and a half minutes a part or more frequent.