Israel utilizes a Counter Battery Radar system along its borders.
globalsecurity.org
Every time a rocket is launched from Gaza, the system records it and reverse plots the trajectory. They know exactly where these rockets are coming from…and they have exact records of every one that was fired.

They also know, through intelligence, what kind of structure (be it a school, house or hospital) the rocket was fired from. Israel has the ultimate documentation of what, where and when each rocket was fired.