count the rings? LOL.
Seriously, there's no way of knowing how old a snake it. You can get a rough idea by the size of the snake, but that's no where near accurate since size depends on feeding schedule + the fact that some snakes simply grow at different rates than others.
My one year old lavender is no where near a big as my amber was when he was one year old. They have the exact same feeding schedule...my amber just grew faster.