What was behind the negative tone of my comment back there: most of my digging has been about topicality, finding out what motivated the creation of a song or tune and identifying what everything in it means. There are no systematic methods for that once it's drifted far enough into the past. You could end up needing any of the sources historians use: historical dictionaries, newspaper archives, genealogies, historical maps, autobiographies, account books, histories of past technologies or army regiments... and some of the sources you'd most expect to be reliable are no such thing.

By the time it got to identifying the politico-religious references in 17th century satirical broadsides I knew I was in it over my head. There are limits.