As I've remarked before, court judgments in art cases are always a good read for anyone interested in art. The background in Mr Justice Newey's 43 page judgment, in the much awaited Vekselberg case, is no exception, and it will be a useful source for students of the works of Russian artist Boris Kustodiev.
The court held that the painting called Odalisque, bought by Victor Vekselberg's company Avrora for £1.7m at Christies, was not attributable to Kustodiev, and so was a fake. It did so applying the civil law burden of proof, which is "on the balance of probability". That means at least 51% sure. It has been reported that Avrora alone spent over £1m on lawyers and experts to get that result, and no doubt Christies' costs were similar. I'm not sure what alternatives there could be, but litigation is a very expensive way to determine attribution, and even then it is only based on a judge's best guess, based on the evidence before him.