Weather radar will give some indication, especially at night, although it detects precipitation and not turbulence. Othwerwise it is completely down to the Captain. Some will turn the signs on at the slightest turbulence while others will allow a slight chop without turning on the signs. Myself I have been on many long flights with the signs turned on in just light chop and have the seen the inconvenience this causes passengers. Therefore when I am the Captain I tend to leave them off in light chop. You have to be careful though in our litigious society. Although now if you leave them on too long someone will complain of the onset of deep vein thrombosis!