The problem is to find out just what proportion of articles are in the niche you want to write about. It's just not something that is immediately obvious. You can look through the numbers of articles submitted to each category on the main pages of many directories, if they include that information.
This is useful, but a graph is better. That's why this article directory is worth looking at. In the bottom right hand corner of the main page it actually has a graph that breaks down the numbers of articles in each category. Of course these proportions would vary between directories. Still, it gives a pretty good rough guide that I think would hold true for a lot of them.