Usually if bub is posterior they feel "all arms and legs" all over your belly because their back is on your back and you can feel everything! That was me with Isabelle and at times I have my suspicions with this little man. If bub is anterior then you should feel most movement all on one side. I find it really hard to determine what is what too and even midwives and Dr's can get it very wrong... I was told at 38 weeks that Isabelle was anterior but she was posterior from32 weeks right up until she was born! At my last appointment the midwife dug around in my pelvis a bit to see if she could feel the head down there, and she "thought" she could feel it but still wasn't 100% sure! Not really an exact science unless an u/s is used, and as you said but bub might move still! The other thing is if you are ike me I spend pretty much no time flat on my back, even on the couch etc.... except for appointments so bub could be in a different position because of that.
HtH!