Assume I got/downloaded (in the past) a bootable ISO file which contains a software which can later be installed onto a bootable USB flash drive.

How can I detect if this ISO file is for Legacy BIOS or UEFI based computers?

Of cause without trying it out.

Can I somehow peek into the ISO file archive and search for a certain file which is special for one of the two boot types (and not the other)?

Which file is it? Is it the same for 32bit AND 64bit?

As you know ISOs resp. bootable USB flash drives can contain dual boot bootloaders for Legacy BIOS AND UEFI.

How can I find out if the ISO is such a dual-boot ISO?

Thank you

use 64bit iso's If you have a 64bit system. There will be EFI directory and/or an efi.img file in it. The efi.img may be either in the isolinux folder or [BOOT] folder. I would say most 64bit iso are setup to boot either EFI or legacy mode

If I remember right, UEFI uses a small dos formatted partition, so I'd guess that if you don't have a dos partition, you won't have a UEFI pendrive.

Yes for a usb that will work with uefi and legacy that is writable, the partition will need to be msdos type drive with a fat32 filesystem. For legacy mode most iso's use syslinux as the bootloader and grub2 for uefi mode.