Generally, you cannot see the sex of the baby clearly and good enough until around 24 weeks... If you do a 3 or 4D ultrasound before hand you might get a better pic..... But usually clearly, the organs are well developed by then and you have better chances of seeing them at 24 weeks.
generally you can't tell before 16 weeks....MOST docs in MY experience will check around 20 weeks when they do the fetal growth scan.
Some see it as early at 15 weeks but most not til around 20 I think on the 3D and 4D you can sometimes tell earlier.