"TOP 3 MOST COMMON TRAITS OF VENOMOUS SNAKES:

1) It is big and fat (all in the USA, except that red-black-yellow coral, are fat pit vipers)
2) It is a rattlesnake, with real rattles on the end of its tail
3) It has elliptical pupils (but I doubt you're close enough to look)"

How To Tell if a Snake is Venomous / Poisonous

One of several references.