Are you using Lightroom CC or Lightroom Classic CC?
Every collection in Lightroom CC either have the smart preview on the cloud or it is synced along with the original image, that you can tell by looking at each image.
In Lightroom Classic, Every sync activity can be viewed in the Lightroom>>Preferences>>Lightroom CC menu.
Also, every synced collection will have this icon on the thumbnail indicating it is synced with the mobile as well.
In both the apps, if images are synced to the cloud then they are will sync across devices.
If you're looking for something else, Do let us know with more details.
You may refer Common questions about Adobe Photoshop Lightroom CC for mobile and Apple TV for more FAQ's
If you are using Lightroom CC, you can alt/option click on the cloud icon to show files which are currently syncing.