The evidence is in how Jonas changes throughout the story. When the novel begins, Jonas is exactly like the rest of the people within his society. He is complacent. A rule follower that doesn't ask questions. He takes euphemisms at their word instead of questioning what the euphemism may be hiding. "Releasing" people for instance sounds very nice and peaceful. Jonas doesn't question what it really means to release someone.
As Jonas gets more and more knowledge by being the Receiver, he begins to question the very fabric of his society. He begins to question all of the rules and practices. Most notably the releasing of people. Jonas learns that it is euthanasia. What I appreciate most about Jonas as a character is that he not only questions his society and its practices, but he takes action. He leaves the community completely and takes his brother with him in order to save his life. If Jonas didn't question anything, he wouldn't have taken that aggressive step toward freedom.