I could tell with both of mine but not really well until 30ish weeks.
The pp who said if you press and the whole baby moves its butt is right. I also could feel the difference in density too. Squishy and wide butt, small and hard (like a cue ball) head. I laid on my back and used my fingers to poke (not hard) around my belly. I was able to tell that dd2 was head down facing left (confirmed by us) and then I could tell when she flipped to face my right side.
I did freak myself out w dd1 because I felt a hard roundness near my hipbone. At 39 weeks I thought it was her head waaaaay out if the pelvis. Turned out to be her knee.