Can you make out if a snake is poisonous or not

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I rarely come across snakes in real life. I tend to see them only on Animal planet, NG, Discovery etc. I really will not be able to make out if a snake is poisonous or not! That is one reason i would hate to face a snake alone. I can recognize a cobra if it spreads its hood or a rattlesnake if it rattles! How about you. Can you identify a snake?