Final answer:
In Philip Roth's novel 'The Human Stain', it is a character named Delphine Roux, not Steena, who finds out that Coleman Silk is actually African American. She discovers this through investigating his background, where she finds letters and documents hinting at his true heritage.
Step-by-step explanation:
The question "how did Steena find out Coleman was black?" appears to reference a plot point from Philip Roth's novel The Human Stain, where a main character, Coleman Silk, passes as Jewish for most of his adult life while his true heritage is African American. The question is concerning how another character, Delphine Roux, or possibly Steena Palsson, becomes aware of Coleman's racial background. In the novel, it's Delphine Roux who learns about Coleman's secret after investigating his background. If Steena Palsson is meant instead of Delphine, she does not find out about Coleman's race within the timeframe of the novel.
In case of Delphine Roux, she finds evidence through letters and documents which reveal Coleman's association with his family and his true racial identity. This dramatic revelation is a pivotal moment in the novel, showcasing the themes of identity, secrecy, and the social constructs of race. The precise methods of discovery are not explicitly detailed in the text but are based on her research and snooping into Coleman's past.