It's one of the first tips editors and published author would tell you. Study the houses, find out what they like and are publishing. Taking a look at their catalogs is one way of doing so. You can see first hand what's being published through them. Many houses are now posting them online, staying green by not printing out physical copies. For example, Random House: http://www.randomhouse.biz/booksellers/childrens/catalogs/. This, of course, makes it easier for us to obtain these catalogs.
And for crazed, obsessed, book loves like myself, these catalogs are a two for one. Information on what kind of books the houses are publishing as well as first hand knowledge on what books I want to add to my Amazon shopping list. You can't beat that.