By tracking the location history of the object. That's actually how much of the art world protects against forgery. Forgers are very good at copying art works, but artists' estates keep a record of what an artist actually produced.

Just to add on to dash2's comment, the art collection world relies on provenance. Most large art museums (in the US, at least) rarely buy from private collectors for this reason. Items from private collectors are usually donated, and if an item is purchased, it is either through auction (usually the big houses like Sotheby's) or from well-trusted dealers.