It starts getting bumpy. The aircraft ahead reports that it has started to get bumpy. Air traffic control reports that aircraft have been reporting turbulence at various levels on that route. Weather charts show forecast areas of likely turbulence.
Aircraft don't have instruments that show clear air turbulence. They do have weather radar that shows areas of significant cloud. They also have instruments that show windspeed and outside temperature, which an experienced and skilled pilot can sometimes use to interpret and predict areas of possible turbulence.