My doctor told me to tell wether it's an actual contraction to feel above your belly button, if it's rock solid your having a contraction. But usually you entire stomach tightens up, you'll notice it'll come and go.
I haven't had any Braxton hicks contractions but from what I've learned, contractions will make your whole belly tighten up and it will be like rock hard. I remember when a co worker of mine was pregnant last year she would have them and her belly would be literally hard as concrete