There are various written sources, including graffiti and poetry, for the use of the tersorium (poop stick), including one mosaic in an ancient latrine reminding users to “Make use of the tersorium” before leaving.

As for finding out about hygiene, ancient poop, believe it or not, is often an important archeological find because of the details we can get from it. Ain’t science grand? There are other matters too, of course - Romans wrote effusively on their bathhouses, many of which survive to this day, as well as poetry admonishing men to groom themselves properly, physician’s manuals, finding nit combs, etc etc.