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?
Answer:
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.
