Click on All Photos, then click of the Filter icon (at the right-hand side of the top-centre search bar). When the filter options appear, click on Sync Status, which will look something like this:

Clicking on "Synced and Backed Up" with show you all the images that are in the cloud in full original format, i.e. taking up space in your allowed cloud storage. "Synced from Lightroom Classic" will show you all images that have been synced from Classic and so are only in Smart Preview format in your cloud space, which do not count against your cloud storage allowance.