I've been trying to teach my wife this and you have to have a method. Staring at the snake in general won't help you. I start at the head and work my way back. So for example, what is the head marking? break that down into, is the head light or dark? is it a solid colour or does it have a pattern? what colour is the eye ? is there an eye stripe and is so what colour is it? then look at the body for colour, pattern, etc, finally look a the belly. If you break it down in that way you will start to see the clues.
but
Knowing the genetics of the parents is always a great place to start if you are blessed with that information.