Is this possible to do and how can I do this with WIN API? I would prefer to use <fstream> but I want to learn the WIN API as well.

Thanks!
Sanction

It depends on the file mode the other programs opens the file.
Try reading with CreateFileA and check the return value.
https://docs.microsoft.com/en-us/windows/desktop/api/fileapi/nf-fileapi-createfilea