Look at other books the publisher has produced. Does the interior formatting look professional? Are all the pages in order? Is the cover art attractive? Are the books of good quality? In other words, does the company produce a professional impression, or does it look like someone threw the books together in his garage?
Request references. And use them.
Talk to people who've used the publisher's services (other than those you've been given as references by the company). Are they happy with the quality of the books? Did they receive all the books they paid for? Did they have any problems getting hold of their books after they were published? Have there been any broken promises?
Choose a publisher that has an arrangement with a wholesaler such as Ingram or Baker & Taylor. That way, even if your books aren't stocked in bookstores, people will at least be able to order them.
If the publisher makes promises, verify them. If the publisher says it has an arrangement with a distributor, make sure it's telling the truth. If there are promises of marketing, ask for sample catalogs, ads, publicity releases and so on, and check their quality (vanity publishers often produce very unprofessional marketing materials). If the publisher claims to be able to get your book into bookstores, check bookstores to make sure you can find the publisher's books.
Comparison shop. Every company offers a slightly different mix of services and options. Costs (and the quality of the finished product) can also vary widely. Make sure the publisher you choose is the one that best fits your needs.
—Used With Permission From Writer Beware (www.writerbeware.org).