when i went in for my confirmation visit i thought i was 10wks but when the doc did a vaginal ultrasound she said i measured at 8wks. the weird thing is that i missed a period in july making me think that i was a little further along but apparently i did not conceive till august. the body is so tricky.
They measure by ultrasound, and how they do that is they draw a line on the screen and measure the fetus/baby head to rump and it gives them a reading for the exact week and day it measures. Also, if you know your LMP you can determine how far along you are. I was unsure by a week or so, so I got a "dating ultrasound" and it determined I was 8 weeks at the time. I hope that answers your questions.
I don't know what to feel for...but when my sister went to her first ob appointment the doctor (which is my doctor so I trust him) felt her uterus and told her that she was about 14 weeks along and that's what the u/s told her too. He's being doing it for quite a while so maybe it's a trick of the trade
They do also feel for your uterus, when I was 10 weeks I was at the midwife and the uterus was very low then and she pressed firmly on me and it was kinda deep, but she took my fingers and pressed them down and it felt round, like a side of a small ball. I think they are able to tell this way by where the usterus is sitting in porportion to the naval.