"A girl is drinking water."
Translation:En pige drikker vand.
Just context or things like usually, at the moment, every day, right now etc.. It's not hard to figure, you'll see it gets easier with time. I imagine it's harder for someone whose native isn't English to learn to differeniate present continuous vs simple (assuming their language has only one form for present) than it would be vice versa. I can't remember having those problems though (I'm Serbian,we don't have continuous/simple, it's just like danish, only one way of saying it). In a way it's like present was simplified, so it doesn't matter that much when actually speaking the language unless you were to translate it. It's actually less complicated :)
*differentiate and my dumbas.s has realised by now we do kinda use different things in Serbian, like doći vs dolaziti, one is ''lasting'' while the other one just happens and it's done. We do use them interchangeably sometimes, so I guess my point still stands, but sorry this is not really helpful :/