If you mean a cartomizer - you'll know. The burnt taste will overcome your lost sense of taste and smell. Believe me.
A standard cartomizer holds 1ml of e-
juice, which is about 20 drops. You can always take the cap off and see if it looks dry - if so you can add a few drops.
Also the vapor production may be less as the
juice runs out. Although I have sometimes been
vaping away and suddenly noticed a slight burnt taste.