Get the current writer's guidelines. A publication's guideline for writers is one of the best sources for knowing what the editor wants. Without going into a trance or utilizing your powers of telepathy, you can find out useful things such as how long an article should be or who is the magazine's target audience. There IS a catch. You need to get the very latest version of the publication's guidelines or you may be shooting yourself in the foot.
Guidelines tend to change with the editor.
Check Writer's Market or some other reliable source to get instructions for obtaining the guidelines from your target publication. Writer's guidelines can also be obtained by email in many cases. Many periodicals are now publishing their current guidelines on the Internet. Check on the following web sites for links to some of these guidelines on the Internet. You'll have to buy a year's subscription to use this site, but it is well worth it.