You could set up a –log-file at –log-file-log-level=verbose, or use live log About → Show logs → Live → Verbose to see what’s going on for given files via “Checking file for changes” output. Example is here with commentary on how it “should” work. Also just use File Explorer or whatever and check modification time.
You could certain test some smaller files in a folder to see if you get the same “upload all files every time” behavior, which would be a serious violation of the way things should work. If modification time changes, a scan of the file can happen to look for any changes, but unchanged data from file should not get uploaded.
Statistics on all of this are in your backup job log. Your values will differ, but here’s the general idea of that:
"ModifiedFiles": 9,
"ExaminedFiles": 448,
"OpenedFiles": 13,
"AddedFiles": 4,
"SizeOfModifiedFiles": 66201655,
"SizeOfAddedFiles": 1164494,
"SizeOfExaminedFiles": 474400640,
"SizeOfOpenedFiles": 67366149,
...
"BytesUploaded": 33761799,
Basically, it normally examines all files present, finds some need examining, and uploads changes found.