When Helene Plotkin purchased a painting of a seated woman at a White Plains thrift store sixty years ago, she bought it because she liked it. A former art student, she admired its colors and brushwork, and the price was right: she remembers it costing under $100. Now, thanks to Ms. Plotkin’s eye, her son’s curiosity, and a little help from the Google chatbot Gemini, her family is more than $250,000 richer. Verified by specialists as an original canvas by noted Scottish artist F.C.B. Cadell (1883–1937), the work sold to a private buyer at auction June 4 for $189,200 pounds with fees. Though Plotkin, now 88, always treasured the painting—which depicts a stylish woman in a dark dress and 1920s-style turban in a modernist interior—she never had it appraised. “I never, never thought about it at all,” Plotkin said in a recent telephone interview with the New York Times , “other than I loved the painting.” The story of how the work came to be identified began...