My machine has an alert button which works sometimes (and it is not lint blocking the indicator). Then it stops and won't sew, but I just leave the door of the bobbin case open if I want to sew to the end. Other times it runs out without warning. There is no notable difference with different weight threads in the bobbin - one can never guess the amount that is left once the light flashes. On my old machine it was usually around 2 metres left.
All of my other machines give no indication, so it is the look or guess method.