Does anyone know if I can check this parameter anywhere?
You can read Graphics.DpiX
to discern this.
Multiply Graphics.DpiX
by 100 and divide by 96 and you will have the percentage font scaling. This is true because 100% font scaling equates to 96dpi.
Be warned that if your application is not marked as DPI aware then when the user sets font scaling to 150% the DpiX
property will report 96. Off the top of my head I do not know whether or not standard WinForms apps are marked as DPI aware.