How did historians figure out exactly when man started making stone tools? How do they know the ones they found were the earliest known made?

Historians don't do this. Archaeologists are the stone tool folks.

They don't know they have the oldest made. All they can do is find some, date them by their surroundings, then say, "These are the earliest tools we have found to date." Something earlier could show up at any time. "Something earlier" has revolutionized the dating of early human activity several times, from the arrival of humans in North America, to how fast they spread into South America, to how early they started making tools at all.