For the most part you'll know, trust me! They are painful. True contractions tend to wrap around to your back as well. Or from your back to your front. There's a huge difference between your belly being hard and a contraction.
Its a different kind of hard, it'll be kind of like cramping. I know for me it starts in my back and.in my lower abdomen (like menstrual cramps in my lower abdomen) and move up through my whole stomach. I can't walk or talk or hardly breath, and while the menstrual like cramps make stay the.other stuff will come and go.