In order to decide which writers opinions are most credible on a particular subject a reader should look for writers who?

Difficult question. Why? Writers have to be published in order to be read. If publishing is controlled by a certain vested interest, you will not be able to read books by writers they disagree with. Therefore you may only be able to choose from the available not get the diversity of opinion you would like. By the way publishing is controlled by vested interests. Look for self published work or alternative publishers if you can find them. Try The book tree.com for example.