Reinherz If you're talking about looking at the comments on last Chapter to judge the paid readers that read that chapter then You can't.
Not all paid readers comment on last page or even on chapters. Out of 100 people, only 5-10 might comment. It just depends on how engaging that chapter is. Some books might have 10 readers out of 20 who feel like commenting while others can have 5 readers out of 30 that comment.
Other than the internal staff, no one can see how much a book is earning
As for the fan ranking, except the top 3, it don't show how much others spent on the book.