sebsmummy the back to back position really can only be picked up on palpation (when midwife feels the tummy)
It could be picked up on scan but babies can move a fair bit so unless they were done routinely in labour it's just down to the midwife to pick it up either through feeling abdomen OR it can be picked up on vaginal examination through feeling different landmarks on the babies head, but that can be difficult
Not an awful lot can be done about it, it's recommended to labour on all fours or sort of 'downward dog' positions before true labour kicks in can help the baby move round, as the back is sort of the 'heavy' bit of baby, so if you're on your front gravity will encourage the weight of the baby's back to gravitate towards your front... and not your back.
It's thought that sitting in reclined positions a lot (ahem, guilty) encourages the back to back position, as again the heavy back of baby will swing round with gravity to settle against your back.
That's the theory anyway!