Corrected entry: Willy is the only one that can verify the golden tickets, but we never see the Russian ticket rejected or the good tickets even looked at by him.
Correction: You're thinking about the 1971 film Willy Wonka and the Chocolate Factory, where it is revealed that the fifth ticket was a forgery.
Correction: The are many ways by which fraud can be detected. The movie never said, nor implied that Willy Wonka was the one who determined the ticket to be forged.