Pilots of passenger aircraft always try to find less turbulent routes - traffic and knowledge allowing. They do lots of weather planning before departure, use radar and eyeballs to do what they can, select routes promising smoother flight and listen to PIREPS (pilot reports) en route.

Nonetheless, the airstream is like a stream of water - hot air rises, winds blow and currents are shaped by land features, so some turbulence is entirely unpredictable. But it's really like a canoe in the water - the airplane will ride the currents, and is built to withstand much higher stresses than those we are likely to encounter.