With boys there is a clear shape of the penis and testes once it had been pointed out to you.
Our sonographer zoomed in on Calebs bits and let me guess and there was no denying what was there. I'd already seen 2 others on an u/s so I knew it was boy bits there.
A 4D u/s leaves absolutley no doubt. If they are able to do a combined scan ask them to check out the bits once you've seen your babys beautiful face.
I've got a pic of Calebs in 3D!