[WildHorseCrash]

How does one get to be a 'peer'?

Roughly, by doing sufficient quality scientific work so as to not only demonstrate sufficient knowledge of the field of study, but also a demonstrated ability to objectively analyze the work of others pursuant to the rules of scientific inquiry.

[PatrickHenry]

Here's a bit of info, not all that much, on Nature's peer review process: publication policies. Click on number 8, "Referee policy." Also, in the left margin, click on "How to get published in Nature." Basically, the editor picks the peers. It says:

Most papers are sent to two or three referees, but some are sent to more or, occasionally, just to one. Referees are chosen for the following reasons:

* independence from the authors and their institutions
* ability to evaluate the technical aspects of the paper fully and fairly
* currently or recently assessing related submissions
* availability to assess the manuscript within the requested time.

I think that every publication has its own rules, but the concept of peer review is pretty much the same everywhere.

[Elsie]

...demonstrate sufficient knowledge of the field of study...

demonstrate to whom?