Is there enough filament to complete the print?

If you don't have a fancy-schmancy printer that tracks filament usage and warns you if you don't have enough filament left on the currently-loaded spool to complete the print, this is how you know. You can even double-check while it's printing, like I do in this video.

This is why you should always keep a kitchen scale and one empty spool of all the filament brands you use.

Low tech but useful. I figured I'd share.