Quoting Dani Bryant-Murray:" how can you tell...like what is the difference in the feeling of baby's head and baby's bum?"
Push around firmly until you feel a hard area. It'll almost feel like a hard lump and it's about a few inches in length. You've probably now discovered the back. Push along the length of baby's back until you either feel another lump or drop off into squishy tummy. Where you feel that second lump is where baby's head is.
It's not 100%. Baby can get into positions where you only feel their back, nothing in particular etc. It's a process of trial and error. I noticed with my second that next to where her butt would be, I'd usually find a foot pressing against the uterus. Sometimes both feet. After getting acquainted with what was what, I eventually figured out foot from fist too.
It takes time, really. I have this mild obsession with interacting with my babies by poking them and shining flashlights on my belly, lol. Interactive fetus, anyone?