How do you know your lantus solarstar pen is Finnish or have no more insulin?

Your Lantus Solostar pen has a clear window that you can see through that contains the insulin. It is clearly marked with black lines and numbers. As the plunger pushes the insulin down with each dose, you can see the level of insulin decreasing through the clear window. Once the plunger gets close to the 20 line, its close to getting finished. Keep your eye on the clear window.