uhhh how did she know???...

I've had a baby myself. But my friend is having twins. And she recently posted to Facebook that her hubby finally felt (we'll call him Bobby) move! He's felt (we'll call him Ricky) move before but this time he felt Bobby. How could she distinguish which twin was moving if she can't see them? They weren't having an ultrasound just sitting on the couch? Anyone with twins were you able to distinguish who was who? How do you even decide who is who before they're out?