Research the Internet to find publishing companies that publish books similar to yours.
Find out if you must pay the publisher a fee to publish your book. Most legitimate publishers don't ask writers to pay anything. They pay authors an advance and royalties. Upfront fees can make you responsible for all the costs related to publishing your book.
Ask if the publisher will market or publicize your book. An unethical publisher may require the writer--you--to take on the expenses of advertising and promoting your book.
Learn how the company markets itself. If it's a subsidy publisher or a vanity press then you may want to avoid them. Bookstores rarely purchase books from these companies.
Check websites like Writer Beware and Preditors and Editors for lists of unethical publishers to avoid.
Hire a lawyer who practices intellectual property law to review any contract offered to you by an agent or a publishing company. She's able to help you avoid unethical publishers and practices.