Mine has in light that comes one when the bobbin is low, but if it gets too "lint-y" underneath, it doesn't work. If the light doesn't come on, I can tell by a tiny difference in the sound of the machine. I don't know how to explain it really, but it just sounds a bit "off". If you have a lot of sewing and the bobbins hasn't been replaced in a while, check it, or just switch to a full bobbin so you don't have to worry about it.