This smudging here on this copy of Shaykh al-Jazuli's Dala'il al-Khayrat (thank you @MmwProject) is quite standard for the Dala'il: the images are rubbed for barakah. https://twitter.com/MmwProject/status/1265313824879583240

The smudging is often ONLY on these illustrated pages (I would need to check through this edition), which gives you a sense of its use.

Prayer books often also have annotations or additions on flyleaves that tell you how the books were used or if they were heavily used.

The quality of the illumination –the illustrations, the incipit frames (whether they're gilded or not) is how I gauge who was buying the book: I am often lucky and I get the repository's name, as well as ownership stamps. My favorites are from masjid, zawiya, or tekke settings.

The size and type of script is really important as well: was someone reading from it to a large group of people?

the type of script and illustration can help me guess the origin of the text if I don't have one but sometimes it doesn't help....

And this thread is brought to you by someone bored of formatting images in a word document. And all my footnotes explaining dala'il 101 to my poor readers.