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The Debian Wiki UEFI page

says

Troubleshooting

How to tell if you've booted via UEFI The Debian installer splash screen will say it's the UEFI installer, and will look slightly different to the equivalent screen in BIOS mode. BIOS boot is done via isolinux/syslinux, but UEFI boot is done using grub.

Later on, the thing to look for is the directory /sys/firmware/efi. If that exists, the system is running in UEFI mode.

There is also a related question on Ask Ubuntu: How can I tell if my system was booted as EFI/UEFI or BIOS?

However, the Wiki page is missing some details. Does the splash screen show up in both text mode as well as in graphical mode? Can someone provide a screenshot of what the EFI/UEFI and BIOS screens look like?

And how would one "look for" the directory /sys/firmware/efi in the Debian installer? Drop to a shell?

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    For the second part, you can open a shell on VTs 2 and 3 (press Alt+F2 or Alt+F3, then Enter). VT 4 shows the installation logs. – Stephen Kitt Jun 15 '17 at 13:37
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It's on the splash screen before you pick text or graphical install. Note the "Debian GNU/Linux UEFI Installer menu" at the top, right under the centered name/version:

Debian Installer in booting in UEFI mode

Here, for comparison, is the legacy BIOS splash screen. Note the lack of UEFI message up top, and the different highlighting of the menu options. Also an extra "help" option:

enter image description here

And to look for that directory, yes, drop to a shell. Easiest by switching to virtual terminal 2 or 3.

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