You can tell in some cases and in some you can't. She probably wasn't lying.
it depends on the baby and the equipment used... there are diff kinds of sonogram machines...new/old... and etc
Before 13 weeks you cannot distinguish female and male genitalia.
It really depends on the pregnancy most arn't able to tell until later on in the pregnancy and other cases it can be told right away...my first we were not able to see what sex until about 24 weeks and my youngest we were able to see at 16 so it really depends.