[dalke]

How do people know that I have (or haven't) been a reviewer for a top journal? Do they have a public list of reviewers?

Of the people I meet at a conference, how do I figure out who has been a reviewer for Nature, and how do I verify that claim?

[mbrundle]

My thoughts exactly. I've never known a journal to do anything to promote the fact that someone has reviewed for them (eg produce a letter on request that states which articles someone has reviewed for them). It'd be a nice and free way of creating value to reviewers who generally work for free, but I don't know any journal who's ever done this.