- The angle of the signature - could be unnatural or badly placed relative to the background.
- Pressure and speed of writing
- Bridges and tunnels in ink
- Rubber stamp "tramlines"
- Texture of the ink
- Autopen dots appearing at start and finish of strokes
Real people write their own name typically by placing the pen to paper and writing in one continuous movement of the pen. An autopen machine on the other hand (sic) touches the paper with a single dot before the pre-programmed template can start; and then ends equally abruptly (with a visible dot) when the machine is raised above the paper. Even if you can't detect these dots readily with the naked eye, they become apparent when magnified.
Autopens will tend to apply consistent pressure of pen to paper resulting in a uniformity of stroke and "boldness" which would be hard to match by the human hand. Conversely you will sometimes see autopen errors e.g. when the paper is moved slightly or the machine is interrupted; in this case unexpected horizontal changes will be generated resulting in "jagged" line strokes which wouldn't normally be found in the real thing.
Finally always remember that when signing their name, humans will tend to search for areas on the page where this is most contrast so as not to mask the handwriting i.e. dark pen on lighter areas, light pen on dark areas. Some autopen machine setups result in badly placed and e.g with many Apollo crew photographs autographs appearing in uniform and equally spaced positions. Note that in some cases 3 signatures can appear in different inks on the same document but all autopen examples nonetheless. Similarly, and confusingly real "in-person" autographs can be placed alongside autopen contemporaries - so you really need to check each sample seperately.