That's not really accurate bc your belly gets hard with Braxton hicks contractions too. Real contractions start more like bad period cramps, low- not on your belly. Then they move and will increase in intensity over time. If you're not sure, walk around, drink water, lie down. If they go away, it was just BH.
I was told when you start to feel a contraction put your hands on yout belly and if your belly is as hard as your forehead that is s real contraction. Some will start off like a cramp but then it gets stronger and more intemse.