How do we know Bruno's family is rich in The Boy in the Striped Pajamas?

Bruno's family is indeed quite well off. We know this because they live in a palatial, five-story house in an exclusive suburb of Berlin. The house has large windows with extensive views, as well as ornamental features like elegant bannisters. The family also employs a number of servants to attend to their every need, such as Lars the butler and Maria the maid. One would certainly need a substantial disposable income to be able to afford such staff.
When it comes to making the big move to "Out-With" (as Bruno calls Auschwitz), it's notable that there a quite a few belongings that need to be packed. Even Bruno, who's still only a young boy, has rather a lot of stuff to take with him. This would appear to provide further evidence of his family's affluence.