This is a question that everyone asks…
I wish I could give you a simple answer that would allow a complete snake novice to instantly recognize the danger or lack thereof for a particular snake. But sadly the only way to know if a snake is venomous or not is to be interested in snakes and take the time to learn. And trust me to become proficient takes a LONG time.
With that in mind their are a few things you can look for that may give you some idea as to wether a snake is venomous or not. However, you should never approach a snake unless accompanied by a herpetologist or professional snake catcher that can insure an accurate ID is made and understands the temperaments of various species. Even non-venomous snakes can inflict a painful bite!
– The shape of the head can give you a clue. Venomous snakes have venom glands which can often result in a more ‘square’ shaped head. Also, non-venomous pythons tend to have large, distinct heads with strongly visible pits. There are a number of exceptions to this however!
– Large bodied, slow moving climbing snakes in Australia are more often than not non-venomous. Again, there are exceptions and many venomous snakes that ‘don’t’ climb have been seen climbing in response to flooding or on occasion just because ;p.
– The presence of fangs. Very difficult to ascertain from a glance but all Australian elapids are front fanged and venomous to some degree – from virtually harmless to potentially fatal. Again there are exceptions, for example the rear fanged Brown Tree Snake (Boiga irregularis) is rear fanged (less of a fang and more of a notched fang-shape tooth to be honest) and mildly venomous.
As I said these are just generalised identifiers and should not be used to ascertain wether a snake is safe to approach or not. The only way to know a snakes danger rating is to study them. A good starting point is identification guides like the one on this website :p
Finally, I have been dealing with venomous and non-venomous snakes for years and they always without exception are scared of people and want to stay out of our way if at all possible. The only time they become aggressive is if they feel threatened and unable to escape. Stay out of a snakes way and it will stay out of yours. Occasionally bites do occur but it is generally because someone has ether stepped on one or attempted to catch or kill it, in fact most snake bites I know of happened to snake enthusiasts!