NovoLog FlexPen - Can the user see how much insulin is left in the pen?

You should look at the scale window on the prefilled syringe, you will need to remove the cap of the pen to view this - when there are 12 units left the syringe should be discarded and a new one replaced. This window is a guide only - it does not tell you exactly how many units of insulin remain.
If you were to inject 7 units when only 3 were left you would not get 4 units of air. You would only get 3 units of insulin.