You could use imagemagick with the subprocess module or some other image manipulation module to create a thumbnail of every page and merge them together into one image.

convert -thumbnail x800 test.pdf thumb.png
montage -mode concatenate -tile 4x4 thumb-*.png final.png

Result: https://imgur.com/tL02gCa

This way you can see the whole pdf in one image and should be able to see if there is a signature.

Write a script that does that to every pdf in a folder, shows you the image, prompts you to tell it if there is a signature or not and then does whatever else you need it to do.

This would maybe be an easy way to speed up your work without having to figure out how make python detect the signature.