"Who would buy your book?" If you can answer, you have begun to identify your target audience. The best titles make that clear. For example, I have an e-book called Devotions for Runners. My target audience is obvious. When a Man You Love Was Abused doesn't leave much doubt about the obvious target.
You don't have to express your target audience in the title, but you need a title that speaks to the people who will buy your book.
For example, a publisher asked me to write a book about the time-for-all-things passage from the book of Ecclesiastes. They released it as Hope and Comfort for Every Season. The book didn't do well and was out of print within a year. The title didn't define the audience—too generic. Even publishers goof.
Once you identify your audience, and you know what other books are available and who would read your book, you also need to express how you will slant your manuscript. The two questions below will help you know how to slant it.
1. Who will buy my book?
2. Why would they buy my book and not someone else's?