PhotoStructure is keeping track of this already, but the UI doesn’t currently have a nice way to check this (but I agree with you, I think it should). In the next release, browse by filesystem is a thing, which would be a good place to add metadata like this.
If you want to peek “under the hood,” you can install DB Browser for SQLite (it’s open source and a great piece of software, I use it all the time), and open your library database (stored in your library directory, .../.photostructure/models/db.sqlite3, and look at the Progress table.
The Progress table is what drives the “progress panels” on the home page. As a sync runs for a given volume, a single row gets updated.
I’ll add a --sync-paths argument to the info tool that exposes this, too (this will be in the next release).
Related: