My preferred, 100% accurate way of determining gender is the aura reading method:
You take a precious stone or metal (i.e. a gold or silver ring, or an amethyst or quartz) and tie it to a thread so it can hang and swing freely from your hand. Hold it above the head of a person or animal, and it will either swing back and forth or spin to indicate the gender of the person or animal. If it moves back and forth, you have a boy. If it spins, you have a girl. After every single reading, you should hold the crystal in your non-dominant hand, make a fist with your dominant hand, and tap the crystal with dominant to clear the energy. You have to allow it to hang for at least 10 seconds to get an accurate reading. It has NEVER been wrong. To make sure your crystal or precious metal is accurate, hold it over someone you KNOW is male or female, like a pet or another person. This ALWAYS gives a spot-on reading, and best of all, you don't have to do any barbaric vent sexing.
Before you wave it off as a bunch of hooey, try it. Believe that the crystal or precious metal you are holding has the ability to determine the type of energy surrounding different people or animals. It is infused with the power of thousands of years, so it is no surprise it can detect and read energy so accurately.
Thank you Fresh Eggs Daily for the idea.