Most periodicals and book publishers will conduct a demographic study periodically to check up on who their readers are and who their advertisers are. This would cover such information as to geographic location, age range, married couples, previously married, singles, number and ages of children, nationality, level of education, etc. Such a study is very helpful to a publisher in being sure what they are producing is meeting the needs of their readers. As a writer, you can write to magazines you want to write for and ask them for a copy of their latest demographic study. They are usually happy to send that to you. Such a study can be very helpful to you as well. What you want to do is match what you are planning to write to fit the demographic of that publication. For example, if most of the readers are in their 20's with young children, you can gauge your article to the needs of that age group. You can also mention that in your query. The more you know about the readers, the better your chances of writing something to fit their needs exactly.