Open Question: Top 400 Adjusted Gross Incomes?

I know there's a website with the top 400 adjusted gross incomes for individual income tax returns. Does anyone know if there's a way to find out how those incomes were earned? I'm not talking about finding out personal information about these people. For example, let's say the top adjusted gross income was $1 million. Would there be a way to find out that that money was earned by, say, writing a book?