Relying solely on a signature for identification is the simplest way to get stuck with a fake. Compare the subject matter, composition, style, signature location, the materials used, size, and formats that the artist usually worked in. Art theft is a fairly common occurrence but it's unlikely that you bought a masterpiece for a song at the local thrift shop. For centuries, artists have learnt from the works of their predecessors and it's not uncommon even today, particularly in Europe, for students to draw and paint from masterpieces in order to study their techniques. It's possible that a talented student couldn't resist signing the masters' name to his work.
True forgeries are hard to detect even by the experts.
One of the most talented forgers in history was Hans van Meegeren. He was arrested after World War II for selling a Dutch national treasure, a Vermeer painting, to Hermann Goering. Look for these signs that your painting has been doctored . Remember, too, that age does not necessarily make a painting valuable unless the work is by a known artist. The work of an artist, particularly if he dies unexpectedly, will escalate if the artist was well-known and his art was expensive and collectible prior to death.