Shakiness: Autopen machines vibrate when in use, causing the signature to appear shaky. A good example of a shaky autopenned signature is Joe Montana's. Nearly 100% of the through-the-mail Joe Montana autographs are done by a very shaky autopen machine. Here is an example:
Abrupt starts and stops: Autopenned signatures usually come to a very abrupt start and stop. This is due to the fact that the machine places the pen straight down onto the item being signed, then lifts it straight up when the signature is finished. This also causes the lines of the signature to have the same thickness, unlike a true autograph. Steve Young's autopenned signature is a good example of abrupt starts and stops (notice the dots, or pools of ink, at the beginning and end of the signature):
Identical signatures: Each signature created by an autopen machine is identical or nearly identical. As you'll notice when you sign your own name, no two hand-signed autographs are exactly the same. Do a search of "Joe Montana autograph" on eBay, and notice how so many of the signatures look EXACTLY the same. That's because the majority of "signed" Joe Montana trading cards on eBay are actually autopenned signatures. However, keep in mind that some famous people use multiple autopen machines or templates.