There is no definitive documentation as to whether Cronus ever found out he ate a rock and his reactions. Historical records show that he swallowed a rock that was rapped in a baby's blanket by his wife Rhea. She was afraid that her son Zeus was also going to be eaten like her other children, who Cronus ate in order to provide safety. Find out more from http://edweb.sdsu.edu/people/bdodge/scaffold/gg/titan.html#Cronus.
He ruled for a long time, but he ate all his children, because he was told one of his children would overthrow him. His wife, Rhea, hid her son, Zeus, and tricked Kronos into eating a rock instead. Many years later, Rhea tricked Kronos into vomiting out his five other children, who joined up with Zeus, who lead them on an attack against the Titans stronghold, Mount Othrys.