When you're pregnant, everybody makes his or her predictions on what you're having. I had waitresses ask me to stand up and turn around so that they could make their best guesses. Once, a homeless man told me I was having a boy. He was so convincing that I almost believed him.
But what most people believe to be the best indicator of gender, other than an ultrasound, is the Chinese lunar calendar. There isn't just one Holy Grail to reference so they can vary on calculating factors but all in all, they are pretty much the same. Based on a women's age at time of conception and the month she conceived, the calendar will tell you if you are having a boy or girl. While it's clearly not foolproof, it was true for me and so many of my friends.
If anything, it's a fun way to guess your baby's gender before knowing the facts.