I guess most reliable may be weight.
But the easiest way of implementing this project may be with this high precision optical sensor and the powerful image processing device that's attached to it, called "eye" and "brain" respectively. As added bonus, this "brain" is very good at fuzzy logic (you old enough to remember that?), so the only programming (also known as "training") would be:
if (you see the bag is almost empty)
flip that light switch to the ON position;