Lower back pain that comes and goes. By the 2nd or 3rd time you feel the pain, you will know this is labor. Sometimes the water will break, also a sure sign of labor, and time to go to the hospital.
Early signs of labor may include a full feeling. A woman who normally never misses a meal may feel like she can't eat anything because of a bloated or full feeling. Also the urge to sit on the toilet can be a sign of early labor. The squatting position is natural and gives comfort to the pregnant woman.
Normally we hear an expression called "the waters broke". It is you placenta getting out of your belly. That and timely contractions means the baby is any time soon.