I've had two babies now, currently growing my third, and I've never been able to tell a butt from a head. When I read about those mamas who can, I waffle back and forth between being in awe, and thinking they are just fooling themselves.
I was sure my second had two heads and was transverse all the way until labor started. Heck, I thought I could feel a head coming out of my side WHEN HE WAS CROWNING!!
I wouldn't get an ultrasound just to check for twins. How would your UC plans change if you found out you were having twins? What about the pressure you would receive from the medical community since you would automatically be classified as "high risk"
For me, I would rather be surprised during labor. That way, I wouldn't spend months worrying for no good reason. I think the risk during twin birth isn't as great as it is often made out to be. And I also believe that instinct takes over when you find yourself in a surprise situation like that, and I am much more comfortable with letting that instinct take over then planning every possiblity. It jives better with my spiritual perspective.
As far as how big you feel, I'm sure you've heard it tons of times by now, but every time is different. I was seven months along before I even looked slightly pregnant with my first. I could feel my second distinctly kicking from 14 weeks on, and with this one I didn't have even the slightest "baby bump" belly until 16 weeks, and since then (now 19weeks) everyday I wake up and and am like "WOW!" I've grown so much.
Anyway, good luck with whatever you decide. =)