Probably by talking to him. It's not hard to tell if a manager has no clue about what you do.
If he doesn't, then he's probably using some silly and lazy metric like "hours in the office" or "lines of code written per day". If he does, he probably took the time to learn about your job and make a more accurate assesment of your work.
Really, as a programmer you are supposed to know lots of things, learn some more and work hard; but the guy above you cannot be bothered to learn some useful skills in order to asses your work? What kind of logic is that?
A good manager still knows what their employees are working on - not necessarily in detail but they should be able to describe the user side of the project comfortably and have some idea about which parts are the most challenging.
