I have a dual boot PC, where the Win10 (uncompressed) partition is encrypted with BitLocker. I was curious about making this test (and also encryption took quite a short time in my opinion), so while running Linux I did this:
# cat /dev/nvme0n1p3 | strings -25 Remove disks or other media. xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx ... some (very few) lines of garbled characters #
The time it took makes sense according to the partition size and type of disk, and about the garbled text, I guess that a there is a small chance that encrypted data could happen to form a short text string
In that partition there are of course lots of plain text files, so is it safe to say that with this test it's 100% sure that all information is encrypted?