How can I check to see differences across my multisites?
That's a task for the Features module. Before Drupal 8 and its superb configuration management the Features module was used to track and deploy changes to entities or settings from local to prod or to your coworkers. Nowadays you can use it to track changes and sync features across multiple instances of Drupal.

Let's say you have two Drupal 8 sites or two Drupal 8 multisite instances and both of them have a content type "Foobar". Then there's Features that lets you export the whole content type into a so called Features module. Which is just another module but also holds the whole config for this "Foobar" content type in it. Now you activate this Features module on both sites or instances and both will have the same content type with the same fields in it.

From now on when you change something in this content type on either instance Features will recognize the changes and offers you to recreate the Features module. Do that and the other instance will recognize the Features module had been recreated and offers you to import these changes.

And that's what you can do now. Go to the instance you consider being the master. Use Features to export the content type in a Features module. Enable this module in this instance and also in the other instance which you think got overridden or which you think is missing the changes and use Features to explore the differences and to maybe sync the changes.


  The features module enables the capture and management of features in
  Drupal. A feature is a collection of Drupal entities which taken
  together satisfy a certain use-case.
  
  Features provides a UI and API for taking different site building
  components from modules with exportables and bundling them together in
  a single feature module. A feature module is like any other Drupal
  module except that it contains additional information in its info file
  so that configuration can be checked, updated, or reverted
  programmatically.