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Use GetDeviceCaps function with LOGPIXELSX parameter to detect what font (large or small) is in use.

procedure TForm1.Button1Click(Sender: TObject);
var
  DC: hDC;
begin
  DC:=GetDC(Form1.Handle);
  if GetDeviceCaps(DC, LOGPIXELSX) = 96 then
    Label1.Caption:='Small font is in use';
  if GetDeviceCaps(DC, LOGPIXELSX) = 120 then
    Label1.Caption:='Large font is in use';
  ReleaseDC(Form1.Handle, DC);
end;