Most of our 20-week scan (DC2's was yesterday) was involved with checking heart function (they do this amazing thing with colours on the screen to show the blood flow between 4 chambers of the heart...so so clever), and that all organs (brain, kidneys, stomach, etc) are visibly present and seem to be functioning. The organs show up as dark splodges so you can start to spot them yourself.
For the rest of it the sonographer was checking key bone structure like the spine, thigh and arm bones, measuring leg bones, measuring size of head and stomach. They count the number of fingers and look for club foot and cleft palate, also check the eyes seem correctly formed. You get a print out showing where your baby's measurements sit according to average.
She also checked the position of the placenta and the amount of fluid around the baby.
This is IF the baby is in the right position...our little so-and-so wasn't, so I have to go back next week for another scan to finish the stuff she couldn't see!
The baby does look bigger than at 12 weeks. The photos we got are only down to the bottom, the legs didn't fit on!
As for the sex being obvious, no it's not. With DD1, DH and I were convinced we saw meat and veg, but apparently the female genitals are very big at that stage. She was a girl, obviously. We didn't see anything obvious yesterday - the sonographer asked at the outset if we wanted to keep it a secret (we did) and so may well have stayed away from the key areas.