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Billy,

Two ways to tell poisonous from non are looking at the eyes and the tails.

A poisonous snake will have oval 'cats eye' whereas non will have round pupil.

The underside scales near tip of tail on non's will be 'broken' (have two
parts) whereas the poisonous variety will appear whole/unbroken.

Downside of both of these is that you have to be close enough to see...
This is where hourly wage employees and interns come in...