Most autopen sigs are not too hard to spot. It helps to understand how an autopen machine works. There are plenty of videos out there showing the process.

Some common areas to look for are the stop-start points. Remember that an autopen is essentially a robot. It is a mechanical device performing a programed task. Because of this, there will be consistent characteristics such as start and stop points. As the matrix wears down, the pen can "shake." This is where the auto starts to look a little shaky or wavy.

Also remember there is no variance in the autograph. The size and shape of the auto will be the same for every sig the machine makes. One example of this is when you see a small signature on a large item. 

I know this is a very brief explanation so I would encourage you to research autopen machines. 

Vintage autos are not just about your exemplar database. Part of authenticating vintage autos is being able to authenticate the inks used and even the item that is signed. For example, it is difficult to fake aged iron gall ink. It just takes years of oxidation to age. That is why you will see fake vintage autos in pencil. In can be more difficult to authenticate pencil sigs since graphite does not age.

@cardgrader - thank you, Andy. I'll do some research on autopen machines. I also appreciate the tips on the ink types. Most of the vintage signed cards I have are from just after the Post-War era, through the early 80s. Most of those cards are signed with ballpoint pen or Sharpies (and similar). But I have a few cards signed with fountain pens, which are beautiful!

The cards in my collection that were signed with fountain pen were already authenticated by reputable companies when I purchased them. But it's nice to have those cards in hand to be able to see the ink and how it flows on the card and interacts with the surface.

Thanks for the tips!