Signed books can be worth quite a bit. How you can be sure that a signature found in a book or document is indeed the real thing?
This is intended to be an overview of the basics. Authenticating signatures is a highly specialized discipline that takes years of experience to master, but when an expert isn't available or practical, there are some tips that amateurs can use to get a better idea of whether a signature is real or not.
There's a deep gulf between the gains of a page signed by Shakespeare and a local author who drops by the independent bookstore to restock their titles once a week. Obviously, that hypothetical Shakespeare signature is one of the most extreme and unlikely examples, but the principal remains. On the very high side of that equation, professional forgers have motivation to use methods and materials that prevent anyone but the most trained and well equipped professional from being able to detect the forgery. The most accomplished of forgers formulate their own ink out of period-correct chemicals including chemical aging agents. Impeccable, but entirely fake, original holographic documents are created from scratch by starting with blank paper from the correct era, sometimes acquired by cutting the blank sheets at the end of old books. The art and science of uncovering those frauds takes far more specialized knowledge than will be covered here.
One honest mistake that people sometimes make with older books is to misinterpret an author signature that is printed with the book rather than added by hand after publication. Authors that became sufficiently famous within their own career sometimes had their signature on the printing plate for decoration. Some of the later works of Robert Frost show that tradition, as do some uniform binding series, such as the Hillcrest Edition of the works of Mark Twain. A few tricks for checking whether the signature you are looking at is a true signature as opposed to something printed with the book:
If the answer to any of those questions was 'yes' you have gotten a little further toward the potential for a real signature.
If you can reasonably be sure that the signature you are looking at was added by hand, the next step is to see how likely it is to be real. Start by comparing the example you are looking at to a known example. The internet provides a great resource for finding examples of famous signatures, but it may take a little work to find a comparison for what you are trying to authenticate. I once was able to find the example I needed under the dust jacket of the same book. The publisher had the author's signature decorating the front board, giving me an incredibly convenient example.
Keep in mind that signatures change as we age. Check when the author likely added that signature and compare it to one of the same approximate vintage if possible. If the author added both a signature and date, it makes things simple. If there isn't a date, the signature may have been added any time between after the date of publication. It pays to do a little quick fact checking on this point. Posthumously published books and editions printed after an author's death are very unlikely to have been signed, so make sure that the signature you are looking at is even possible before spending too much time on the finer points.
The next step is to look at the signature closely. A good source of light and a loupe or magnifying glass are important here. Look at the lines in the signature. Does the ink make sense for the time of that signature?
Is the signature fluid?
If your signature passes all of those basic tests, you may want to next go to the professionals.
There are many signature experts in the business, but keep in mind there is no universal professional certification or licensing. The experience and reputation of the professional is your guideline. Also, keep in mind any statement of authenticity provided is that individual's expert opinion. Many of the same principles for professional authentication of a signature are the same as getting a professional appraisal. If the goal is to sell the item, starting out with talking to professional sellers in the field about how much they can offer you may save you enormous amounts of time and money over marketing it yourself.