I have always heard they look for three lines to be sure- two lines doesn't mean boy, but it's hard to confirm a girl with just two as there could be something hidden. It's not an exact science either way though- my sister in law had 4 ultrasounds- for the first three she was told it was a girl for sure- ultrasound number 4 revealed it was a boy- it did end up being a boy.
That's a first for me on hearing lines . This is my second baby and I'm having a girl. With both ultrasounds i got the gender and not lines the ultrasound technician said boy with my first and girl with my second :/