How Do I Know If A File Has Been Successfully Synced To iCloud Drive?
OS X 10.10.2 now has graphical upload progress bars:



When things get stuck, or you have a slower upload, there is a brctl tool that ships with Yosemite to monitor changes in all iCloud backed documents. Open terminal and type the following command:

brctl log --wait --shorten


On macOS Catalina 10.15 use the following command:

brctl log -w --shorten


It will show you a brief summary of the sync changes both up and down. The wait causes it to run continuously until you close the window or issue Control+C to quit brctl process. If you are familiar with the grep command line tool, you could filter the results to just show upload progress, but most iCloud accounts are quite low volume in terms of changes, so your upload of many files will likely dominate the results.

https://www.icloud.com/#iclouddrive

Also, the empirical way to tell if a file is synced is to log in to iCloud drive on the web interface and verify a file and/or folder is up there in the clouds.