If it is a newer septic it should have an alarm, if enough water gets in the float raises and the alarm sounds. If it is an older septic it may not have an alarm and water may get in and then slowly leak out into the drain field. Of course if to much water were getting in you would think it would back up into the house through the toilet. Our at least this is how it was explained to me when I went through my septic tank nightmare on my new house that was having the alarm randomly go off for no apparent reason. Turned out to be a nick in a wire.