I've read several times that when you lie on your back, you can/should/will be able to feel your uterus moving up, out of your pelvic region. I have no idea how to tell. What "feeling" am I looking for? Just a tightness? I have some extra belly chub, if that matters.

Baby Boy born sleeping at 20 weeks.
Re: What does your uterus feel like?
ETA: at 14 weeks for me it felt like it was about the size of an orange, and I best felt it just above my pelvic bone. Still felt like a little ball, though. ;-)
At 14 weeks, it can still be pretty low. If you have some extra padding there, you may need to try pushing the fat up a little and then press near your pubic bone to feel the top of your uterus, which will feel kind of firm. In the coming weeks, as it moves up more, you should be able to feel it a little more easily when you're on your back.
I've been having a hard time telling the difference this week. In weeks 12 & 13 it was super obvious though! Really low my abdomen went from soft to hard. Now at 15 weeks I'm having a much more difficult time but when I feel around about half an inch below my belly button I can feel a slight difference.
I think a lot of it depends on your body shape/how much pressure you're using. Try sort of walking your fingers down from just above your belly button to an inch below to see if you notice a difference.