Fortunately I've had a lot of practice genrefying when I used to work for Realms of Fantasy, the short fiction magazine. I was one of their editors and part of my job was helping authors to classify their stories so they market them to the right publications. And I really enjoy figuring out what genres can be applied to a book, believe it or not. Applying genres to a book is really similar to listing its subject headings.
It isn't always foolproof, but I ask myself the following questions when I'm trying to genrefy a book, and they usually do the trick for me: