In response to this question I wrote this code.

function IsSameBitmapUsingScanLine(Bitmap1, Bitmap2: TBitmap): Boolean;
var
 i           : Integer;
 ScanBytes   : Integer;
begin
  Result:= (Bitmap1<>nil) and (Bitmap2<>nil);
  if not Result then exit;
  Result:=(bitmap1.Width=bitmap2.Width) and (bitmap1.Height=bitmap2.Height) and (bitmap1.PixelFormat=bitmap2.PixelFormat) ;

  if not Result then exit;

  ScanBytes := Abs(Integer(Bitmap1.Scanline[1]) - Integer(Bitmap1.Scanline[0]));
  for i:=0 to Bitmap1.Height-1 do
  begin
    Result:=CompareMem(Bitmap1.ScanLine[i],Bitmap2.ScanLine[i],ScanBytes);
    if not Result then exit;
  end;

end;