It depends on alot of things - the position of the baby as Emma said and also the "policy" of the place doing the scan.
Some public hospitals won't tell you even if they can see. Also some of the xray/ultrasound places will not give you a definite answer. They'll say "80% chance" even if they can see.

Private clinics on the other hand are generally pretty good. The place I have had all my scans is a specialist fetal medicine place and they have no qualms telling you what the sex is with an honest assesment of how sure they are about it. With this baby and with my daughter I was told they were absolutely positive and since I could also see it for myself I had no problem with that.
But my friend who went to an xray/u/s clinic was told "yeah we're 80% sure it's a boy" even though she could clearly see his bits on the screen (and yes he was a boy).