Check if there are any historical records or news articles that match the events or themes presented in '21'. If it's a well - known true story, there may be some documentation out there that can help identify it. Sometimes, if it's based on a local event, local archives or community knowledge could be a great source.
Another approach is to analyze the details in the story itself. For example, if there are specific time periods, locations, or character traits that seem unique. You can then try to cross - reference these with known true events. If it's a story about a particular industry, like the casino industry in the movie '21', you can research real - life events in that industry to find a match.
One way could be through research. Look for any promotional materials or interviews related to the '21' project. The creators might have mentioned the true story it's based on in those.