Can you tell if a snake is venomous from casual examination?
Not really with certainty. Non-venomous snakes often mimic venomous varieties, because it's a lot safer to be venomous!
This non-venomous gopher snake looks exactly like a rattlesnake, especially with the tip of it's tail tucked up so you can't see it. It has bulged out it's cheeks so that it's head appears triangular, and it has assumed the confident striking pose as well. If snakes were eligible, this snake would get an Academy Award.
This non-venomous black snake is rattling it's tail against the ground and making a lot of noise. It doesn't sound precisely like a rattlesnake, but certainly close enough that you might think it was one--especially if you couldn't see the snake clearly.
The non-venomous scarlet king snake has a bright red-yellow-black striped pattern, almost exactly like the highly venomous coral snake. The primary difference is that the king snake has red-black-yellow-black stripes and the coral snake has red-yellow-black-yellow stripes...for the common species in North America. If you only get a glimpse of the snake or if you're more focused on running away, it's easy to mistake the two.
The exceptions are some harmless snakes which have very distinctive appearances that aren't shared by venomous snakes, such as ring-necked snakes (they are technically venomous, but it's only a problem if you're very, very small--like if you were about the size of a small frog or mouse). You can usually identify a ring-necked snake at a glance, but most snakes are not so easy.
Also, even a non-venomous snake bite can be unpleasant. The northern watersnake is not venomous, but it often looks like a copperhead and is notoriously short tempered and willing to bite. Even though their bite won't kill you, they can draw blood and generally make you unhappy. (Don't get between them and the water, it makes them feel threatened.)
The safest approach is to treat all snakes as though they will bite you and a bite will be unpleasant. Give them room when you can, and when you can't (such as you've woken up with one on your chest) move slowly and non-threateningly and let the snake depart of it's own accord. None of these snakes want to harm you or have anything to do with you--you are too big for them to eat.