No radar, not even doppler radar, can see or predict every tornado.
Still, the National Weather Service estimates that today's doppler radars have improved warning accuracies from 30 percent to about 80 percent!
The bigger the tornado, the more likely it is to be detected. Certain wind movements within a storm provide signatures that point to tornado development.
Without doppler radar, early warnings of tornadoes and severe weather would be far less effective.