One sign could be inconsistent statements. If the girl's story changes from one interview to another, it might be a sign of it being fake. Also, if there are elements in the story that seem too outlandish and not based on any real psychological or social phenomenon, that's suspect.
Well, first of all, if the producers of the Dr Phil show have a history of creating fabricated or semi - fabricated stories for ratings, that's a big red flag. Secondly, if the girl's story doesn't align with any known psychological or cultural frameworks. For instance, if her supposed 'black identity' is just based on very flimsy or illogical reasons like just a passing fancy or a desire for attention. And if there are no other sources or people in her life who can confirm or give more context to her story, it makes it more likely to be a fake story.