I think that impossible is not right word, for example NTFS Windows dedup can at least tell us dedup ratio and saving
Anyone can also run ddpeval.exe to get evaluation of dedup saving for some folder
Example: Using the Deduplication Evaluation Tool (ddpeval.exe) to Calculate Space Savings Gained before deploying the Windows 2012 Deduplication Feature | Pipe2Text.com
It’s portable exe and it works on W7 and higher.