The head moves independantly of the body, wheras the butt dont, so if you gently feel on each side of the lump, and its wrigglable.. its prolly the head. Kinda hard to explain, my midwife explained it all to me and had me feel at the same time so it was easier to understand lol, maybe ask your midwife to teach you at your next appointment?
The midwife did say though that even they get it wrong sometimes, so while its fun to have a feel and try and work it out.. dont take it too seriously, if you know what i mean!
Ive just cleaned the kitchen.. about to get down on my hands and knees to do the floor LOL !