You know, when my baby is head down, I haven't been able to feel the head with confidence, either. I mean, I can feel something hard down there right above my pubic bone (which is where my OB always checks right before she confidently announces that the head is there), but I can't get a really good feel and tell for sure it's the HEAD. When the head is up, though, I can tell for sure. It feels like a big cue ball, hard and uniformly round on all sides--I can kind of grab it and cup it in my hand. When the bum is up, it's only partly round, but then leads into something that feels big and flat (the back) or smaller and flat (a leg). Sometimes I've been momentarily tricked thinking that a knee is the head, if it's sticking out a certain way, but I can tell it's a bit too small, and when I grab it he generally jerks it away quickly, and he doesn't (can't?) do that with his head. HTH.
Mom to sweet DD 8/08, and adorable DS 10/20/10