darkaether- the coral snake is an elapid (family that has king cobras and mambas) and one of the key features of elapids are the large scales on their head, not to mention that they are all horribly venomous. this doesn't fit for vipers, who are also terribly venomous, but tend to have small head scales, like this guy:



i think that you were probably told that piece of information by someone who only had half the story. you can certainly use head scales to tell if a snake who looks an awful lot like a venomous snake is venomous or not IF you know the scale patterns of the venomous snake.

there's no hard and fast rule here.