I have this issue for years and it happens all the time. There is no way to find out which files or folders bird is stuck on.
I have around 1 TB of data in my iCloud Drive and the last time I killed the CloudDocs folder (as mentioned here as a possible solution) it took 40 hours until the files even appeared in my iCloud Drive! I cannot afford that.
Apple, you can do so many amazing things, but a basic file sync is too heavy to lift?