Ninhydrin reacts with a mixture of amino acids and turns purple. Could proline be one of the amino acids? How could you find out if the...?

Proline does react with ninhydrin. But, when proline reacts with ninhydrin, it produced a yellow color. That yellow color could easily be covered up by the purple color of a mixture of other amino acids. To know whether proline is present, you would need to separate the amino acids (maybe by thin layer chromatography or paper chromatography) and see if you detect proline in the mixture.