How can I tell when Lantus pen is empty, it's supposed to have 100 units. I have used 5x's 10 units?

Most Lantus pens have 300 units in them. It's 100u per mL, and the pens have 3 mL in them. Also, if you pull the cap off and look at the clear part, the black part of the syringe itself will slowly descend as you use up what is prefilled in the pen. When the black piece gets to the bottom, the pen is almost empty. Also, it's almost empty if it will no longet set to your dosage.