Proving that a painting is what someone thinks it is can be a daunting task, especially when there is evidence that it may have been painted by an old master whose name would make the canvas worth millions. This week, a once-disputed Rembrandt will go under the hammer in New York, and whoever buys it will inherit quite a history along with the art. The painting, which was discovered in 1972, wasn’t certified as a Rembrandt until nearly a quarter-century later, and its story underlines the important advances that have been made in art analysis over the past few decades.