I have Intel DP43TF motherboard and the only option related to UEFI is "UEFI Boot" (Enable/Disable). Like on this screenshot I've found. But whether it is enabled or disabled, I can't see if it has any effect.
Here is how I know it doesn't boot in UEFI mode: I boot from Arch Linux boot image (UPD installation CD), run
modprobe efivars, but
/sys/firmware/efi doesn't exists.
UPD The question has nothing to do with Arch Linux. Tell me another way of checking if it boots in UEFI mode, and I'll give it a try.
UPD For one reason or another it now boots in UEFI mode, but than it hangs. So either I missed separate entry in BIOS's boot menu for running Arch Linux in UEFI mode (that is, my flash drive has now two separate entries in the menu), or Arch Linux had poor support for UEFI back then. Or still has. Or I might be experiencing some hardware issues.