Identifying Autopens
Humans tend to choose clear light open areas of an image to place their signature, avoiding dark or cluttered areas where the signature will be obscured. Autopen operators don't necessarily pay so much attention as to where the machine puts the signature. The Apollo 11 crew litho below is a good example of poorly-placed autopen signatures.
It's important to bear in mind that although poor placement is an indicator of a possible autopen, good signature placement does not mean the signature is necessarily real, as a careful autopen machine operator would certainly try to place the signature as naturally as possible.
A good example of this is with the Apollo 13 flight crew litho, a dark image with limited space for natural signature placement. Signed examples of this crew image are very commonly autopenned, with one or two of the signatures often carefully placed in the globe at the bottom of the picture.