How do pilots foresee turbulence?
There are a few ways that pilots are aware of potentially turbulent areas.  

Eyesight
The most obvious way is just by looking outside and observing the sky.  Large billowing clouds, called cumulus clouds, indicated pockets of unstable air (the clouds are rising because the air under them is as well).  If the pilots must fly through these clouds then its a safe bet that there will be some turbulence.

Weather Radar
Just like using your eyes, except the radar can see further through haze and other clouds.  Typically this is useful for finding embedded thunderstorms, but it can also be useful to find areas of potential turbulence.

Communication
Pilots talk.  Both to each other and to air traffic controllers.  En route controllers frequently ask pilots for "PIREPS" (pilot reports), to build an accurate picture of the flight conditions at different altitudes.  Often commercial aircraft will request to change altitudes or deviate around weather/ turbulence, so it is in the best interest of the controller to know ahead of time where the bad flight conditions are, and have a game plan of how to route traffic.  This makes it much easier for the controller to route traffic, rather than getting request after request from individual aircraft.