How can I tell if my wife is telling the truth or lying?

In this episode Dr. Betters answers the question, "My wife is depressed. She blames me for her depression. She also has cheated on me, and is currently doing so via text message. We are separated, but we live in the same house. She doesn't know if she wants to work on our marriage. She says she has demons in her head that she needs to work out before she can love herself, or me. Is she telling me the truth or lying to me?"