Start by visiting some large bookstores. Take your notebook and a
pen. Copy the Tables of Contents of books that treat the same subject
matter that your book does. You will want to make your book
significantly different from other books that cover the same topic.
If your book is the same as other books on the topic, no publisher
will show interest in buying it. However, don't be discouraged if you
find many books covering the area in which you intend to cover. Many
books mean that this area is very popular. For example, publishers
bring out endless diet books each year. And there's room for yours,
Aim for at least three to five points of difference. This doesn't mean that you have to come up with all new information. In fact, presenting completely new information is impossible. Sticking with our diet book example, there's only one way to lose weight, and that's to take in fewer calories than you expend. Authors reveal this ghastly news to their readers in many ways. Therefore, it's how you present the material that counts. If you can show readers a new way to diet, and you can prove that your method works, you're in, with a hot seller on your hands.
Next, drive to the library. Ask the librarian for Books In Print. This multi-volume set of reference books lists all the books currently available by author, subject and title. Your library may have the books, or it may have the BIP CDs. If your library's BIP is on CD, get a printout of all the books in your subject area.
Don't faint if you see an ultra-lengthy list! Several years ago when I was assessing the market for a book on time management, BIP spat out ten-plus pages. I got all the books that sounded as though they might be similar via inter-library loan, and none resembled my book at all. So the fact that you find many books means little other than that this subject is popular. This is a good thing!
Next check out Forthcoming Books. FC should be available at your library right near BIP. FC lists all those books that publishers will release in the next six months.
You'll want to have the books which are the main competition for your book on hand if possible. You don't have to buy them all. You can borrow them from the library, or if they're listed on Amazon.com, you can use Amazon.com's clever "Look Inside" technology, so that you can scan the contents pages of competing titles.
Amazon.com is your next port of call. Type the subject of your book into the search query box, and you'll get a list of books which touch on your subject area. Print out this list. Having the list handy helps you when the time comes to pick a title. Read the descriptions, and all the reviews of any books that sound as if they might be similar to yours.
Now you've finished surveying the marketplace as it stands for your idea, take the time to write a brief report on what you've discovered. This report is for your own use. Do this right away when it's all still fresh in your mind. It's important to do this, because when you talk to your editor or agent, you'll want to have all the information on the market situation handy. Your report doesn't have to be long. A page will do.