During a gender scan Sonographers generally look between the legs of your baby.
There are specific views that need to be seen. One would be what is known as a potty shot which shows an image of the baby from below or a view as if you are in a potty looking up. It's helpful to check from the side (like with the nub theory) and from the front as well.When checking the gender it is always best to check from a few angles as one view alone is not sufficient to give a thorough opinion and neither is looking at one still image. It is quite important to be as thorough as possible because without knowing it, once parents are aware of the gender of their baby they tend to start thinking of their baby as their son or daughter.
It's really important to remember that even with a test that look at the chromosomes for example an amniocentesis, CVS or non-invasive prenatal testing the accuracy of the result is only approximately 99.9% and ultrasound cannot offer this level of accuracy, although it does an extremely good job.
Most people today understand the limitations of ultrasound, more so than only 5 years ago but don't be put off because you can't get a 100% guarantee. No Sonographer would ever deliberately give you an incorrect opinion and if a Sonographer is able to check from as many angles as possible they have done everything they can to ensure that they have checked properly then that is guarantee without making sweeping claims for the purpose of gaining your custom. Honesty is always the best policy!
Ultrasound facilities can look at numbers for the last year perhaps to offer an idea if you would like a percentage, but it is absolutely down to views, experience of sonographers and the position of your baby.