Oh that would be frustrating. I asked my midwife just how she 'does' it and she says 'practice!' I'd imagine with some women it might be more difficult to tell based on their body type, but not being a midwife I am just guessing. I would expect an expirienced midwife to be able to tell.. but I am no expert on midwives.
My midwife says she thought she knew based on feeling the position in my abdomen. She tried to show me how a little bit. She instructed me to see if I could feel a rounded part at the top and bottom of my uterus. I think she said if its hard and doesn't wiggle much its probably the head. If the rounded part at the top wiggles back and forth its most likely a butt. Not sure if that helps you any.
She did an internal exam 37 weeks to see if she could 'feel' the head. She indicated that this would more accurate than feeling through the abdomen. She says it was hard and boney like a head... so I assume she is pretty confident in her assesment. If your baby is in any other position, though, I suppose the exam wouldn't tell you much.
She hasn't checked it again since, and that was the first and last internal exam she's ever done for me. I think she may do another exam tomorrow, though, for my 39 week appointment.
Just to be safe maybe keep doing things to encourage the baby to turn or stay in the proper position, like pelvic rocking.
Karen Mommy of McKenna 2003 & Alysson 2004 Expecting an early Christmas Present