But how can we be sure that is what the author intended?

That's a really interesting point to raise. In short, I don't think we can, especially if the author has passed away. 

Looking at the text though, there are definitely hints pointing us to a certain message, an idea, almost like an encrypted message if you like. Sometimes, reading what critics have thought about the piece of work can help, but what they say shouldn't be the definitive answer. What makes literature interesting is that there can be many answers, as long as you can explain logically how you came to that interpretation. But if two people have very different ideas about what something means, that in itself is also very striking. We can look at what is making them think differently, or what this contrast in interpretation can tell us about the composition of the work in the first place.