How to test whether Windows 10 was installed with UEFI?

Open the System Information Utility (Win Key + "R" > "msinfo32" > OK)

In system summary, look for "BIOS MODE".

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Alternatively, check Disk manager. If you have a partition on your disk for "EFI system partition" then you are using UEFI. If you only have system reserved and C:, its probably BIOS.

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Hope one of these helps

You can also use Windows Panther directory logs:

type C:\Windows\Panther\setupact.log|find /i "Detected boot environment"

Extracted from: http://www.sysadmit.com/2017/04/windows-uefi-o-legacy-bios.html