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.