Starting from Windows 10 Fall Creators Update (version 16299.15) and OneDrive build 17.3.7064.1005 the On-Demand Files are available for users (https://support.office.com/en-us/article/learn-about-onedrive-files-on-demand-0e6860d3-d9f3-4971-b321-7092438fb38e)
Any OneDrive file now can have one of the following type: online-only, locally available, and always available.
Using WinAPI how can I know that the file (e.g. "C:\Users\Username\OneDrive\Getting started with OneDrive.pdf") is online-only file?
After years, I'm still using FILE_ATTRIBUTE_RECALL_ON_DATA_ACCESS attribute described here to determine if a file or a directory is completely present locally or not.
Microsoft docs says the following for FILE_ATTRIBUTE_RECALL_ON_DATA_ACCESS:
When this attribute is set, it means that the file or directory is not fully present locally. For a file that means that not all of its data is on local storage (e.g. it may be sparse with some data still in remote storage). For a directory it means that some of the directory contents are being virtualized from another location. Reading the file / enumerating the directory will be more expensive than normal, e.g. it will cause at least some of the file/directory content to be fetched from a remote store. Only kernel-mode callers can set this bit.
There are some advantages of FILE_ATTRIBUTE_RECALL_ON_DATA_ACCESS:
And as it described in this answer, there are still some interesting undocumented attributes which can provide additional information about cloud files.
Note: I didn't accept Jonathan Potter's answer because I mentioned FILE_ATTRIBUTE_RECALL_ON_DATA_ACCESS attribute in comments and started using it a year earlier than he updated his answer.