Telling the difference will come with time, how ever with all the different species that look similar it can take a scale count to tell the difference.
Pythonian: Not every species of snake has their own forum, some have to share. ie there's multiple kinds of bloods, a multiple morphs of corns, never mind all the different kings, rats and milks. Many boa's look the same to the untrained eye. Venomous covers a huge number of species from cobras to rattlers to mangroves...