One big issue with cloud syncing services like iCloud is knowing when they’re finished uploading files you’ve added, or that have been added elsewhere. Often you’ll need to know this because you’ll want to turn off a computer, safe in the knowledge the files are in the cloud.
With Yosemite you can use the following command to get a clue, which should be issued in a Terminal window (which you’ll find in the Utilities folder of the Applications list):
brctl log --wait
This shows the iCloud syncing log file. What you’ll see is a little cryptic but if you see something like the following:
[note] 12062.038 [2015-06-13 22:57:06.357] brc.sync sync.up BRCSyncUpOperation.m:422 sending 199 items to the cloud for com.apple.CloudDocs [note] 12074.324 [2015-06-13 22:57:18.643] sqlite.serverTruth zone.server BRCServerZone.m:748 received 199 edited items and 0 deleted items from the cloud for com.apple.CloudDocs
… then it’s clear that syncing is still taking place. Another clue is that the list will keep updating as new entries to the log are added.
However, if the list is static for several minutes — not updated — then you can be reasonably sure that syncing has completed.
To reassure yourself you can also use the Activity Monitor app (again, it’s in the Utilities folder of Applications) and click the Network tab, before looking for the bird and cloudd entries in the list to see if they’re sending or receiving data.