You can check someone's earnings in the spreadsheets available and most of the signature campaigns are transparent with their payments. Anyone can see the spreadsheets of those campaigns to know that how much the participants earn for their contribution.

It really doesn't matter that how much someone earns for their contribution the only thing matter that how much valuable information a user contributes to the forum and the campaigns payments are a kind of rewards for that contribution.

If you also want to earn some money from signature campaigns in future then you should focus more on contributing to the forum than on knowing what others are making for their contribution.