How to tell if you are UEFI booting into Windows

Open an administrative command prompt and type

bcdedit /enum

You should get something similar to this:

Windows Boot Manager
--------------------
identifier              {bootmgr}
device                  partition=G:
path                    \EFI\Microsoft\Boot\bootmgfw.efi
description             Windows Boot Manager
locale                  en-US
inherit                 {globalsettings}
default                 {current}
resumeobject            {c60ccebc-b3b0-11e2-a58b-f01faf015989}
displayorder            {current}
toolsdisplayorder       {memdiag}
timeout                 30

Windows Boot Loader
-------------------
identifier              {current}
device                  partition=C:
path                    \WINDOWS\system32\winload.efi
description             Windows 8.1 Preview
locale                  en-US
inherit                 {bootloadersettings}
recoverysequence        {c60ccebe-b3b0-11e2-a58b-f01faf015989}
recoveryenabled         Yes
isolatedcontext         Yes
allowedinmemorysettings 0x15000075
osdevice                partition=C:
systemroot              \WINDOWS
resumeobject            {c60ccebc-b3b0-11e2-a58b-f01faf015989}
nx                      OptIn
bootmenupolicy          Standard

Look for the path variable.

If it ends in .efi - you are UEFI booting
It will end in .exe if you are booting in the older way