When I was quite young, a snake man guy came to my school in Karratha. He showed us a bunch of native (Australian) snakes and someone asked how to tell if a snake was poisonous.

The guy said that if the snake's head is smoothly connected to the neck (there arn't any bumps), then it's poisonous. And, if that's not the case, then it's not poisonous.
So, for some of my whee-fun ASCII art:

Not - poisonous:
       /     \__________________________
     /     o     
     \     o  __________________________

   /  o
   \  o

After looking at a few pictures on Australian websites, this seems to be, for the most part, true.
One excecption that Mr Snake Man pointed out, though, was the Death Adder. I don't know whether this holds for other elapids too though...