Check the HKLM\System\CurrentControlSet\Control\PEFirmwareType registry value to see if the PC is booted to UEFI or BIOS mode. Note: you may need to run wpeutil UpdateBootInfo to make sure this value is present.
reg query HKLM\System\CurrentControlSet\Control /v PEFirmwareType
This command returns 0x1 if the PC is booted into BIOS mode, or 0x2 if the PC is booted in UEFI mode.
wpeutil UpdateBootInfo
for /f "tokens=2* delims= " %%A in ('reg query HKLM\System\CurrentControlSet\Control /v PEFirmwareType') DO SET Firmware=%%B
:: Note: delims is a TAB followed by a space.
if %Firmware%==0x1 echo The PC is booted in BIOS mode.
if %Firmware%==0x2 echo The PC is booted in UEFI mode.