I had a writer ask me this question in my slush queue, after I rejected his story. (I told him the story wasn’t what the editor was probably looking for and it had some other issues, which I gave him my opinion on, briefly.)
I thought other writers might get something out of the answer, so I’m going to post it here, too. One of the reasons I started reading slush (for free) for Apex was that I wanted to learn how to write better by seeing thing from a different perspective; this is one of the things I’m working on.
Determining what stories a market will publish is one of the Great Arts of writing short fiction. I have not yet mastered it myself. The main way to do it–and I’m not speaking as a slush editor here but as a writer–is to read stories in the magazine.
In addition, each editor at each magazine is different, and they change. Many editors are also writers; I also recommend reading their work as well. Often the editors will leave a trail of breadcrumbs in the form of “Writers who we admire” or some such in an interview. There are a lot of good interviews containing that question housed on Duotrope.com.
This is not something you can do all at one time, as you prepare to send stories out to each market, unless you only have one story you’re submitting! This is something to do over time in order to become a professional-level writer.
I’m still working on it myself.
In short – you can read as many submission guidelines as you want, but only first-hand experience with stories can give you a solid idea of what will sell.