It's pretty easy to feel when the uterus is all tight. About softball size and just as hard. If the uterus is slack it feels more like a balloon. Kinda like you can poke a finger in and make an indention where as if it's tight and you poke, the whole thing moves.
Make sense?

It doesn't hurt as much when you do it because you have more feedback. Not that you neccessary stop if more is needed, but you can keep off of the other organs.
Being able to use gentle pressure may be not be universal. Just gentle pressure or even changing positions and mine cramps right up. Also can't stress enough how important getting the baby nursing is. It releases oxytocin which contracts the uterus... Just like nipple stimulation will sometimes get labor going. Some midwives will give a shot of pitocin immediately after delivery to do the same thing.
My midwives usually check once or twice to make sure I know what I'm doing.
My first was born in the hospital and my mom leaned over while they were getting the baby suctioned (possible meconium which wasn't

) and whispered to me that if I would massage my own uterus I'd like it much better than letting them. Boy was she right!!! The nurse came over "let me check you honey" and promptly ground away like she was trying to screw my belly button through my back.

Why all the trauma on all the other organs that live in that space?

Next time I said I think it's tight and the new (and darling

) nurse says "I think your right!" after a brief feel. Whew!!!!
Yeah, rather let them massage than bleed to death, but if I can help it I'll take care of my body myself - thank you very much!