[EricH]

A rangefinder, or several of them, mounted above the bin. When all (or one) of them show below X distance, light the light. Might work really well if the bin could be somehow shaken to settle the dust.

Weight sensor would work well, and simply--provided that you know the weight of a full bin. ("Load Cell" is what you'd be looking for.)

Density sensors don't really exist that I'm aware of. You could rig one with a weight sensor on a known-volume bin.

Depending on how the sawdust enters the bin, it might be possible to rig a flow-rate sensor on the entry/exit route(s) and run some calculations.