Sally i can assure you that you will know the difference between the aches that you are having now (which are perfectly normal) and the start of contractions. With all my pg's they all started the same,an ache like a period pain i suppose right along my lower pelvic area. Its very hard to explain if you havent experienced it before.
My aches turned to pain quiet quickly because neither of my kids mucked around. Keep in mind everybody's body is different so you may have contractions start but then seem to stop only to start up again later iykwim.

How far are you anyway? For memory those pains you are explaining are the bub moving into the pelvic area but it was like that for a few weeks before i was due. Its uncomfortable towards the end thats for sure and i dont miss it at all, Lol.
Good luck with everything and dont stress. You will be overwhelmed with love before you know it and your life will change like you could never imagine. They are well worth the wait.