How do I re-charge beijing smart card if I don't have enough money on it to make a subway journey?

For instance I wish to travel from the Airport to Dongzhimen (Line 2), change at Xizhimen (Line 4) and get off at Xinjiekou, how many Yuan would I need on my transportation card? How would I know how much money I would need for a particular journey? Is there a notice at the Station to work out the fare and how could I find out how much I had on my Card?

Thank you.