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;