Ash - posterior labour is generally back labour, and you will know it. With both my labours, each contraction was all in my back, and I honestly thought my back was going to break. Both my girls turned as their heads crowned. I didn't realise this, the midwife told me as it was happening (as I was pushing mind you!).
With Nina, it was only that I asked that the midwife told me she was posterior. With Emily, the midwife told me as soon as I got to hospital, and she encouraged me to be in certain positions to try and turn her around. None of which worked mind you.