...with one caveat: How long has the system been in place?

We have services that were built and are in the background because they are effectively finished (for now.)

An example would be a service that handles a third party integration. It might get a commit a month once it's built.

From my limited perspective, I look at two warning signs: Do changes require changes to more than two services on a regular basis? Ideally, the majority of changes are limited to one service. If you're propagating changes up a stack, it is going to be a slow change. (The whole point of microservices is that once set up, you can move faster, right?)