When you’re worth just shy of $4 billion, being cheated out of a couple of million dollars in a suspected wine fraud might seem inconsequential.

But not to William I. Koch.

Mr. Koch, the industrialist whose achievements include winning the America’s Cup yacht race in 1992, has undertaken a self-described crusade to “shine a bright light on this fake wine business.” He has filed lawsuits around the country against accused counterfeiters, and said he has spent $25 million investigating fraudulent wine.

He has hired glass experts, label experts, glue experts and cork experts to examine the wines in his own cellar.

On Friday, Mr. Koch’s crusade took him to Federal District Court in Manhattan, where he took the witness stand against Rudy Kurniawan, the renowned rare wine dealer whose charm and expertise, prosecutors say, helped mask the fraudulent sale of counterfeit wine for millions of dollars. Mr. Kurniawan was arrested and charged in 2012 with mail and wire fraud and faces up to 20 years in prison.