If you don't know if the person is lying ... then you can learn equally from each ... as long as there is a 'truth' embedded in the story.
I remember listening to 'fables' on the radio on weekend mornings when I was 6 years old ... e.g. 'the boy who cried wolf' ... they weren't true stories but they embedded universal truths ... very powerful stuff for a child.
I'm a hypnotist by profession ... I was an average hypnotist until I went on a course once and spent 9 intensive days that taught you how to tell stories ... true or made up. The point was to take the 'change' or 'moral' or 'lesson' that the listener needs to learn and weave a story around it.
To begin with the more powerful stories were 'true' stories from your own experience because you could put your own emotion into them - you had lived them and they resonated. You soon learned how to generate stories that had emotional resonance even if they were made up.
Why is Shakespeare still studied in many English speaking schools today - Macbeth, Hamlet, The Shrew etc? These are stories that are made up ... but they still resonate because they are full of universal truths about he human condition.
Now, many of my 'sessions' with clients are just me weaving a story with a metaphor for change embedded in them ... no swinging watches :-), no eyes closed while I 'program' the client (a myth anyway) ... just people listening to a story ... and I am a much better therapist as a result.
In short - the forgotten art of story telling is not about whether the story is true ... it is about whether is reflects and embeds a universal truth!