Here's an example from a wikipedia article:
"The work was probably carried out beginning in the early 860s and completed around 866–67. This is based on a dedication in the book identifying as frater (brother) Wulfad, who was made a bishop in 866, making it unlikely that Eriugena would have used so casual a reference after that elevation."
What do you call this part of history, or what would be a search term for this, and what resources exist as an overview of this topic?
These “dating practices” use hints and other dates to establish something called “terminus post quem” or terminus ante quem” fancy Latin for the limit after or before which something couldn’t happen). The folks in medieval studies who like this sort of stuff are often looking at manuscript transmission or medieval historiography. The two are often intertwined, but you can choose your starting term based on whether you’re looking for the traditions of a particular historical narrative (historiography) or the actual physical copies of the source of that narrative (manuscript tradition/transmission).