The Shulchan Aruch says one has to make a bracha acharona on foods if one eats the size of a kezayis within kedei achilas pras. How did Jews throughout history know if they had an olive size? There are books out there that have pictures of different foods as a kezayis but what does one, who doesn’t have this book, do? Should one just estimate? Is that what Jews have done throughout history?
Even today, many people don’t have books, and they rely on themselves to gauge the required amount. Throughout history, Jews were brought up with a keen sense of halachic observance, and I imagine that the estimation of the kezayis amount was ingrained from a fairly young age. Note that a kezayis is not much, and in the great majority of circumstances somebody eating will be consuming more than a kezayis, so that he would not need to make a fine estimate.