I wouldn't completely dismiss ISOs not downloaded from Microsoft servers as non-genuine.
Consider this scenario:
I download an ISO from Microsoft servers then save to my PC later I upload it to my cloud services for safe keeping (or perhaps to save HDD space). A friend of mine needs the same ISO and I link them to my cloud drive where they download it. In this case this does not invalidate the genuinity or intergrity of the file.
However I randomly get a link to some cloud drive and download an ISO (no link given by a friend this time) there are 50-50 chances the integrity is compromised or not.
In this case a good way it to compare the hashes. You need a tools to verify file integrity using MD5 and SHA1 hashes:
Alternatively rather than searching Microsoft’s MSDN pages looking for the correct checksum, it’s easier to use the Adguard (a third party tool to download Windows 7, 8.1 and 10 from Microsoft servers)
Hope this helps