I knew with both of mine.

With DD I knew she was a girl the minute I saw the two lines. It was positively eerie. I just knew in my gut. We never even discussed boys names I was so sure.

With DS I felt at the start he was a boy but I wasn't dead sure like with DD. When I was about 14 weeks we asked our OB to have a look at the sex and he told us he thought it was a girl. It just didn't feel right to me and we didn't tell a soul (we told everyone with DD) because I just thought it was wrong.
Sure enough at the 20 week scan the crown jewels were there for all to see