Hmmm. I think with experience in palpation and births, you begin to get a fairly good sense of baby's size/weight. The hands and brain just start to make associations over time. I'm never concerned about getting a close estimate of weight on baby; in pregnancy, I'm more concerned about feeling whether or not baby is growing from visit to visit. Closer to term, all I can say is that I've gotten really good at knowing which babies are 6lbs or more; and sometimes I'm able to guess birth weight within an ounce or two--always within a pound! But have never been wrong about a baby being 6lbs or more (or less) at term. I don't have a belief that it is important to have a good estimate of the baby's weight prior to birth...I just want to know if it's 'big enough for dates' (or not) as pregnancy progresses; 'big enough for being born healthy' (or not) when labor happens (speaking very generally of course).
I'd say that my method of weight estimation is about as good as U/S, same margin for error in late pregnancy and at term.