UEFI is a feature supported by motherboards. Check the manual for your motherboard to see if it supports UEFI-BIOS.
Check if the file /sys/firmware/efi exists. If it exists, then you booted using UEFI.
https://en.wikipedia.org/wiki/Unified_Extensible_Firmware_Interface
https://help.ubuntu.com/community/UEFI
https://help.ubuntu.com/community/UEFIBooting
http://www.howtogeek.com/56958/
