The main way to find out what color your eyes truly are is by examining the iris and the eye itself. Using natural light and a mirror can be a great way to do so, as natural light is going to give you the clearest vision of your eye and iris. While both green and hazel eyes have green in the iris, there is a major difference when examining the color that sets hazel eyes apart. Those with hazel eyes have a higher melanin concentration, which is why there are specks of brown on their iris (via Owlcation). Those with high melanin concentration are also more likely to get freckles and more easily sunburn.

The reason so many people get confused with hazel eyes in comparison to green eyes is simple: The perception of color can shift. While our eye color does not necessarily change, the way in which we see the eye color can. Have you ever seen someone with blue eyes wear a blue shirt or dress and you think, "Wow, that dress makes your eyes pop?" The same goes for hazel eyes. When you have someone wearing something green, the green in their eyes stands out more. However, on the contrary, if they wear brown, the brown in their eyes is more predominant. 

Therefore, it's not the eye color itself that changes, but how we see the eye color that does.