Under normal circumstances, a process known as "source monitoring" should help us differentiate between real and imagined events. However, as demonstrated by Elizabeth Loftus and her colleagues, imagining an event or reading an account of an event is sufficient to increase a person's confidence in the truthfulness of a memory.
In many cases, we are unable to "prove" whether a person's memories are true or not. In the case of possible childhood sexual abuse, the legal system has its own methods for evaluating testimony. For the psychologist, the recommended strategy is to do what is possible to relieve any distress on the part of the client, which does not require the therapist to come to a conclusion about its source.