The first thing that happens when you turn on iCloud Photo Library on your iOS device or Mac is that all your photos and videos in the Photos app get uploaded to iCloud. Depending on the size of your photo library and your Internet connection, this can take a long time to complete.
My iPhone had around 1,500 items between photos and videos. Uploading those to iCloud Photo Library took some days to complete. While the uploading is happening, in the Photos app, at the bottom of the Moments view—not in Collections or Years views—you get an indication of the progress with the number of items still to upload. This number seems to decrease in groups of 20 or 40—instead of one by one—making it appear as if the progress is stuck, which is particularly noticeable when a video in your library is being uploaded.
But the majority of my photos are in the Photos Library on my Mac—which as I write this is still uploading—with over 10,000 photos and close to 350 videos, ranging from low quality photos of small size, to RAW files from DSLR cameras and videos of many lengths. In total, Finder reports the size of the Photos Library on my Mac to be 105 GB. Like Photos for iOS, the Photos app for Mac also reports the progress of the upload at the bottom of the Moments view, showing the number of items—photos and videos—still to upload, their total size, and how much of that total size has been uploaded. For example, it will show something like "Uploading 7,162 Items (2.07 GB of 33.92 GB)". This information, including a progress bar, can also be found clicking on Photos > Preferences > iCloud. Also there is a very handy button to "Pause for one day" the uploading and downloading of items. When paused, the button changes to "Resume" and the progress bar displays instead the number of items in iCloud, uploaded by other devices, that have not been downloaded to your Mac.
But there are some things that I have noticed about all this. For example, the number of uploading items and their total size—the 7,162 and 33.92 GB respectively from my example—seem to include all items still considered for upload and their size, even if they were previously uploaded to iCloud by one of your other devices. So the number of items actually uploaded might be less than that. The other number, the one showing the size already uploaded—the 2.07 GB from my example—seems to become increasingly inaccurate. My guess is that when Photos is uploading a file and for some reason it gets interrupted, the uploading of that file might start again from zero, but the progress doesn't go back, and the file size ends up being accounted more than once. So I've learned not to trust that number. It is only good to see that things are moving on.
But, while this whole uploading process is happening, I was wondering how to know which items from the Photos Library on my Mac have been uploaded, and even better what exactly is being uploaded, in case things seem to be stuck? I have found two ways to find what is being uploaded. One of them is less precise and just give you an idea of which items are being uploaded, the other shows you exactly which files are being uploaded. In my own anecdotal experience, the photos and videos seem to be uploaded in chronological order, from newest to oldest, which helps particularly with the first method below.
The not-so-precise method requires that you have assigned names to the faces found in the Faces album. One of the nice things about using iCloud Photo Library is that those faces now become searchable in Photos on your iOS device, but of course, only for the items that have been already uploaded from your Mac to iCloud and downloaded to your device. So for example, my son is in a high percentage of my photos. If I search for his name in Photos on my iPhone, the oldest photo found—conveniently shown at the top of the list—gives me a good indication of the latest items being uploaded. The search results also show the geographical location of the photos, their date, and a "Show All" button that when tapped shows the other photos in the same group. As I said, not very precise but lets you know where the progress is.
The other method requires starting Activity Monitor on your Mac. On the Network page, if the processes are sorted by Sent Bytes in descending order, towards the top of the list—if not at the very top—there should be a process called "cloudd". Select it and click on the Info button (or command-I). Select "Open Files and Ports" and from the list displayed you should find one or more entries that start like this "/Users/
In my case, it means that I still have about 10 years worth of photos to upload.