Thanks to Stl_Cardinals_Fan on Another Site:
Very often autographs are not authentically signed by the individual in question. Autographs come in several forms:
A mechanical device that "signs" flat objects such as photos, cards, letters, etc. A writing instrument is inserted into the mechanical armature and the device "signs" a name using a preprogrammed pattern (matrix) that simulates the person's real signature. Almost any type of pen can be used by an Autopen. (Note: Autopen is capitalized because it's the brand name of the device.) Good Autopens can be difficult to detect, but there are some tell-tale signs that an Autopen has been used.
"Shaky" signatures - A signature that appears unnaturally "shaky." This happens when the Autopen armature vibrates as it signs.
Exact matches - Autopen signatures match each other exactly. This does not occur with real signatures. Sign your name a million times and no two signatures will be EXACTLY the same. If two signatures match exactly they are Autopen signatures. Of course you need 2 samples of the signature to conduct this test. Publications like Pen & Quill, Autograph Collector and Autograph Times frequently publish known Autopen patterns. Buy and save these magazines for reference. There are also many online sources where you can see Autopen patterns. If you're checking out scans on someone's web page and their signature matches yours, guess what? You both have Autopen signatures. Importantly, size of the signature and pen type don't matter. George Bush typically sends Autopenned photos -- some signed with a fine point marker, others signed with a thicker Sharpie. At first glance they may appear to be different signatures, but they match exactly. It's just a different type of pen that was used in the Autopen machine. Also, be aware that some celebrities -- most notably Nolan Ryan -- use several different Autopen patterns. it is also important to note, that an Autopen machine can place a signature anywhere on the item. Just because you have two identical signatures in different spots, does not mean it's authentic.
Lines are all same thickness - An Autopen holds the writing instrument at a 90 degree angle from the writing surface. This means that, in an Autopen signature, the lines are all the same thickness. When signing, a real hand holds the pen at a 50 - 70 degree angle to the paper. This makes some of the lines in the signature slightly thicker than others. This phenomena can be seen most visibly when a thick tipped pen is used and there are loops in the signature.
Abrupt starts and stops - When a human hand writes, there are often "drag" or "lift" marks left where the pen was raised from the paper. This can often be seen best at the end of a signature where the last letter "tails" off. With an Autopen, this doesn't happen. An Autopen lowers and raises from the surface straight up or down at a 90 degree angle. This means Autopen signatures often start and end abruptly leaving a dot of ink where the pen lowers and raises. The George Bush Autopen signature is a good example.
Some people known to send Autopens are:
Most politicians in office
Nolan Ryan (several patterns) (will sign authentically for a fee)
Many current and retired astronauts
Walter Cronkite (seems to have several patterns)
Samuel L. Jackson
Preprints are usually fairly easy to detect. A preprint is simply a photographic copy of an original signed photograph. On a preprint, the signature often appears to be below the surface gloss of the photo and the signature is often very "flat." To test, hold the photo up at an angle to a light source -- a real signature is written on the surface of the photo and should have a different level of reflectivity than the rest of the surface. A preprint will blend right in with the surface because it's underneath the surface gloss. Obviously, preprint signatures will match exactly, AND the signature will be in the same exact place on each photo. Depending on the background color and contrast in the photo, some preprints are easier to detect than others. The Jim Carrey "Spank you very much" is a notorious preprint that has fooled many people.
Some people known to send preprints are:
Many Retired Politicians
Most Star Trek actors
Secretarials are perhaps the most difficult to detect. A skilled secretary can emulate signatures very well. In some cases, a secretary may have been signing for so many years that even experts can't tell the difference between real and secretarial. Sometimes a secretarial signature may appear to be more "deliberate" and slowly written. Also look for loops -- sometimes secretarial loops are looser and more "feminine" than authentic. (This assumes it's a female secretary signing for her male boss, of course.) If there is an inscription, does it match signature? Sometimes secretaries let their guard down when writing inscriptions and you can tell the inscription handwriting doesn't look like it came from the same person who signed the item. .
Some people known to send (or have sent) secretarial-signed items are:
Perhaps the crudest of all fake signature types, stamped signatures are simply signatures applied with a rubber stamp. They are usually easy to detect. Look for these signs to detect a stamped signature:
Uneven ink distribution. The ink may "pool-up" in part of the signature.
Bleeding or smudging may occur when too much ink is put on the stamp.
In a real signature, you can often see a "brush stroke" in the direction that the pen moved - especially with felt tipped pens. A stamp will not have this. This ink is simply laid down on the surface, there is no directional stroke.
Of course, stamped signatures will all be identical.
Celebrities known to send stamped autographs include
A forgery is when one forges a signature for the purpose of selling it to another under false pretenses. Unfortunately, there are many forgeries on the autograph market today. Some forgeries are excellent and can fool the most knowledgeable experts, however, most forgers are as unskilled as they are greedy and unethical. To detect forgeries go through this checklist:
Compare it to samples known to be authentic. While everyone's autograph varies slightly from signature to signature, there should not be major differences in any part of the signature. For example, a person whose real signature has a pointed "A" is unlikely to use a rounded "A."
Compare the letters within the autograph in relation to each other. For example, if Babe Ruth's real signature always has the "B" and "R" twice the height of the other letters, be wary of a signature where the "B" and "R" are the same size of the rest of the letters.
Also look for the tilt and size of the signature. Most people are very consistant in the size and tilt or angle of their signature. Stay away from a signature that varies greatly from known authentic exemplars.
Be wary if the signature looks like it was written slowly and deliberately or it looks like the pen stops in the middle of the signature. This happens when someone is trying to imitate or trace the autograph from another source.
Make sure there are no anachronisms in the autograph. Forgers often make stupid mistakes. Is the paper and pen type appropriate? I've seen "Einstein" typed signed letters printed with a laser printer. Einstein was dead for 30 years before the laser printer existed! I've seen "Humphrey Bogart" signed with blue Sharpie. Bogie was dead for many years before the blue Sharpie was introduced. Astonishingly, forgers sometimes misspell the name!
Don't trust Certificates of Authenticity (COAs). They aren't worth the paper they're printed on. Anyone with a printer can make a COA, and if they're going to forge an autograph, they won't hesitate to make a COA.