How can I see what photo the photos app is currently downloading from iCloud?
Several things.


Open https://www.icloud.com/#photos and you can see / download the full resolution photo.
Then for the same photo - choose File -> Export -> Export Unmodified Original and compare the metadata and size difference between what came from your App and from the "truth" directly from the cloud. (Dragging or normal exports seem to be a different resolution than either the internal reduced version or the full, unmodified original in my experience.)


As to monitoring thing, that's easy too. Open the terminal app and use this command to monitor all cloud file operations up to and down from the cloud.

brctl log -w


Like many terminal command line tools, you need to send the exit signal or close the window/app to get it to exit. Control and C is that command. Two follow on items, the Photos 1.1 is a bit better, so you might upgrade your OS and you can also pause the download if you wish in the iCloud preferences for Photos. It lets things stabilize so you can see what you have and not be testing as things change/download from the cloud.