Fair or Foul?

[hawk65]

I was base umpire in C position. Batter hits a shot down 1st base line. Plate umpire didn't pick up the ball until it was well beyond the 1st base bag and didn't have any idea whether it was fair or foul. I had no idea from my position so could offer no help and he of course has to make a timely call. Is the default call "always fair if I didn't see it" or "always foul if I didn't see it" or do you just guess and live with it? Or what clues do you use to make an educated guess?