However, there are at least two other copies of the same painting in museums, and they claim that theirs is the original. The experts disagree.
Authentication is a difficult but interesting field.
You can become an expert in any field through study, observation and experience. But to start out you need to buy from people you can trust. Never buy from someone who does not give a money back guarantee. Then take your purchase to at least one other knowledgeable person for a second opinion, and return the piece if it does not pan out to be as represented. Common sense will carry you quite a distance.