As a rule, lower back pain just goes along with being heavily pregnant. Both my previous bubs were anterior at all my later checks and I still got lower back pain. I wouldn't worry too much yet cause they still move around lots.
As for being able to tell... I was told this bub was anterior at my check up last week. I am really only feeling movements down my sides and right at the top and bottom of my belly, nothing at the very front. If bub was posterior I assume I would feel kicks and punches more to the front.