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If you have a system with multiple disks and multiple EFI System Partitions how can you determine which one was used to boot the Linux system once the system is booted if they both end up booting the same kernel and root partition?

Pete
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You could simply run sudo efibootmgr -v and see which boot entry has been used to boot the system. You'll see something like HD(1,GPT,PARTUUID) and the PARTUUID is what you're looking for.

You can use blkid or sfdisk -d /dev/device to see your partitions PARTUUIDs.

( There are lots of UUIDs, not to be confused: What is UUID, PARTUUID and PTUUID? )

Thanks, telcoM, for correcting me.

Artem S. Tashkinov
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    On my system, I have boot options referring to two different disks, both beginning with HD(1,GPT. So it seems the number is not reliable. Instead, you should pay attention to the GUID specified after GPT. This will exactly match the partition unique UUID in the GPT partition table, viewable with commands lsblk -o +partuuid or blkid. Section 3.1.2 of the UEFI spec v2.9 says: "The boot manager must use the GUID or or signature and partition number in the hard drive device path to match it to a device in the system." So the GUID would be the primary identifier, not the disk number. – telcoM Aug 15 '21 at 13:44