As part of the TSMBIOS project, I needed a method to distinguish when Windows was installed in Legacy BIOS or UEFI mode. The solution was provided by the GetFirmwareEnvironmentVariable function.

The msdn documentation states

Firmware variables are not supported on a legacy BIOS-based system. The GetFirmwareEnvironmentVariable function will always fail on a legacy BIOS-based system, or if Windows was installed using legacy BIOS on a system that supports both legacy BIOS and UEFI. To identify these conditions, call the function with a dummy firmware environment name such as an empty string (“”) for the lpName parameter and a dummy GUID such as “{00000000-0000-0000-0000-000000000000}” for the lpGuid parameter. On a legacy BIOS-based system, or on a system that supports both legacy BIOS and UEFI where Windows was installed using legacy BIOS, the function will fail with ERROR_INVALID_FUNCTION. On a UEFI-based system, the function will fail with an error specific to the firmware, such as ERROR_NOACCESS, to indicate that the dummy GUID namespace does not exist.
.

So the Delphi code to detect such condition will be something like so

{$APPTYPE CONSOLE}

uses
  Windows,
  SysUtils;

function GetFirmwareEnvironmentVariableA(lpName, lpGuid: LPCSTR; pBuffer: Pointer;
  nSize: DWORD): DWORD; stdcall; external kernel32 name 'GetFirmwareEnvironmentVariableA';

begin
  try
    GetFirmwareEnvironmentVariableA('','{00000000-0000-0000-0000-000000000000}', nil,0);
    if (GetLastError = ERROR_INVALID_FUNCTION) then
      Writeln('Legacy BIOS')
    else
      Writeln('UEFI Boot Mode');
  except
    on E: Exception do
      Writeln(E.ClassName, ': ', E.Message);
  end;
  Readln;
end.