Автор: http://www.swissdelphicenter.ch

uses
  Printers;

type
  TMargins = record
    Left,
      Top,
      Right,
      Bottom: Double
  end;

procedure GetPrinterMargins(var Margins: TMargins);
var
  PixelsPerInch: TPoint;
  PhysPageSize: TPoint;
  OffsetStart: TPoint;
  PageRes: TPoint;
begin
  PixelsPerInch.y := GetDeviceCaps(Printer.Handle, LOGPIXELSY);
  PixelsPerInch.x := GetDeviceCaps(Printer.Handle, LOGPIXELSX);
  Escape(Printer.Handle, GETPHYSPAGESIZE, 0, nil, @PhysPageSize);
  Escape(Printer.Handle, GETPRINTINGOFFSET, 0, nil, @OffsetStart);
  PageRes.y := GetDeviceCaps(Printer.Handle, VERTRES);
  PageRes.x := GetDeviceCaps(Printer.Handle, HORZRES);
  // Top Margin
  Margins.Top := OffsetStart.y / PixelsPerInch.y;
  // Left Margin
  Margins.Left := OffsetStart.x / PixelsPerInch.x;
  // Bottom Margin
  Margins.Bottom := ((PhysPageSize.y - PageRes.y) / PixelsPerInch.y) -
    (OffsetStart.y / PixelsPerInch.y);
  // Right Margin
  Margins.Right := ((PhysPageSize.x - PageRes.x) / PixelsPerInch.x) -
    (OffsetStart.x / PixelsPerInch.x);
end;

function InchToCm(Pixel: Single): Single;
// Convert inch to Centimeter
begin
  Result := Pixel * 2.54
end;

procedure TForm1.Button1Click(Sender: TObject);
var
  Margins: TMargins;
begin
  GetPrinterMargins(Margins);
  ShowMessage(Format('Margins: (Left: %1.3f, Top: %1.3f, Right: %1.3f, Bottom: %1.3f)',
    [InchToCm(Margins.Left),
    InchToCm(Margins.Top),
      InchToCm(Margins.Right),
      InchToCm(Margins.Bottom)]));
end;