Changez strikes me as a fairly reliable narrator. The reason for this is that he is quite willing to say a number of things that would (and do) absolutely anger the American man he is speaking to. For example, he admits that when the attacks on September 11, 2001 took place, there was a part of him that was glad, not because thousands of innocent people died, but because America—full of people who previously thought of their country as invulnerable—learned that it, too, can be struck down or brought low. He knows that such a confession will not make him sympathetic to his audience, and he is willing to make it regardless.
He is also quite open about his relationship with Erica, including the sexual component of their relationship, and it also—at times—makes him look bad. When Changez describes how he encouraged Erica to pretend that he was her dead boyfriend, Chris, in order to sleep with her, it certainly seems as though he took advantage of her vulnerability, and the fact that he continued to try to pursue her even after she made it clear that their relationship was over also does not reflect well on him. In short, Changez is willing to share even the circumstances of his life that make him look bad, and this makes it seem as though he is quite honest and reliable.