There are several ways to do this:
- See if they've posted preferences anywhere.
Does the editor have a blog (like me) or is he/she specifically requesting a certain type of manuscript (also like me)? Because that's the best way to know your manuscript is going to someone who will actually look at it if the person says they are actively seeking something.
- Check the various children's writers' boards.
Just because you haven't found the blog post or announcement doesn't mean someone else didn't find it. Look around at places like SCBWI or Verla Kay's site. I know my picture book submission guidelines have been put up there.
- Ask around.
Perhaps you know someone who has worked with that editor before. What did he/she tell your friend about his/her preferences?
- Read things the editor has worked on.
This is the method that is least precise, but probably the one you'll most often have to rely on. After all, editors do work on projects they haven't acquire and that might not be to their tastes. I know I have. However, this will be your best bet on getting to know your editor when you don't have any personal or online intel. As I've said before, to find out what the editor has worked on, simply go to any book that is in the same genre as yours and look at the acknowledgments page. A majority of the time, the author has thanked his/her editor. Voila. You now have a potential submission target.