Method for assessing polymer and process for producing polymer

The present invention provides a method for detecting, easily and simply, iron and other metallic foreign particles which are contained in a polymer. This method comprises: a step for passing a polymer solution through a filter; a step for bringing a color-forming fluid which contains potassium hexacyanoferrate (II), potassium hexacyanoferrate (III) and an acid into contact with the filter; and a step for detecting foreign particles on the basis of the color developed on the filter, and measuring the number and/or sizes of the foreign particles. The pH of the color-forming fluid is -0.40 to +0.75.