If you are asking how SFML calculates the framerate an application is running at, the simple answer is: it doesn't, you have to do it yourself.
If you are asking how to calculate the framerate in an SFML application, the answer is: just like in any other application, count the number of frames every second.
How you output a value is a totally different question. Time measurement has nothing to directly do with FPS, FPS is derived from time.
If you want to learn how to use timers and the time constructs in SFML 2.0 look here:
http://sfml-dev.org/tutorials/2.0/system-time.php