Well I only have a son, so I don't have experience with girl ultrasounds, but maybe not seeing anything is the point. With my son, there was no mistaking it.. you could see it there.. no problem... So maybe if there isn't anything to see, you know it is a girl??
I have no idea how they check either but I'd say you should trust the tech. I'm guessing you wanted a boy? LOL But go with the u/s tech's answer because when I had one the tech said boy and I refused to believe it because I wanted a girl the second time and told everybody that there is a 10% chance she's wrong so I was still expecting to be surprised. I have two boys now :-)
far out i have no idea how they can tell she or he is tiny!
good luck mate :)