You usually cannot tell for sure by appearance alone
Assume any unknown snake may be venomous
Check local species guides for your area
Look for a triangular or broad head
Look for slit-like pupils
Look for heat-sensing pits between the eye and nostril
Look for a thick body with a narrow neck
Look for a rattle on the tail
Look for bright warning coloration in some species
Look for a single row of scales on the underside of the tail after the anal plate in some species
Look for a more defensive posture such as coiling and raising the head
Do not handle the snake to identify it
Keep a safe distance and use binoculars or a zoom camera if needed
Contact local wildlife authorities or a snake expert for identification