All I've ever done is tell them who originally published the story and when. Sometimes I've led with, "This story was originally published in..." and sometimes with one sentence about the story before getting into where it was previously published. Doesn't really seem to matter where you put the information, so long as it's there, the market to which you submit actually accepts reprints, and the letter is brief. I've used something remotely similar to this:
X-story was originally published in name-of-magazine back in, Month/Year. I see that fill-in-the-blank-name-of-magazine publishes reprints and I think that your audience would get a kick out of this story.
Elaborate upon, modify, or toss out the window as desired. As an editor, I didn't spend much time looking these things over unless there were outright mistakes in them. I was always more interested in what the first page of the story looked like, in particular the first paragraph, and the first sentence. Chances are somewhere between 90 and 99% positive that if an author can't make the first line work, much less the first paragraph, the rest of the story won't work, either.
