How Can I tell If A Snake Is Venomous?

You usually can't. The only way to tell if a snake is venomous is to identify the snake. There are no general rules, and names are as misleading as snake photographs.
Any Brown Snake, Western Brown or King Brown can be olive, greenish, gray, copper or golden coloured. Many a harmless python has been killed for being brownish in colour. Other harmless Australian snakes died for being mottled or stripy. Not every striped snake is a Tiger Snake!
Juveniles can look very different from adult snakes. Size is no indicator at all. They're all small when they hatch.
Don't think small Australian snakes are harmless! A grown snake can control its venom and will often inflict a warning bite without injecting venom. This is not always true of a juvenile snake.
I call pythons harmless as they aren't venomous, but they still bite when annoyed. These bites can cause nasty infections and give you more trouble than a bite from a venomous snake.
Visit wildlife parks. Take a good look at the snakes you see - if you can do so safely - and look at the number, size and the shape of the scales, particularly on the head. Compare it with your book. Ask someone who knows.
The more snakes you see the easier it gets to spot the differences and characteristics. That's how I learned about Australian snakes