One way is through the work of archaeologists.
Archaeologists are time detectives.
They dig through earth as if they were digging through layers of time. At each layer, they look for evidence of human culture in the remains of objects left behind by the people who made and used them. Certain questions help archaeologists to identify and understand the importance of these objects. Some questions take a long time to answer and require the help of historians who study the written materials of past cultures. As they find the answers they are looking for, archaeologists can begin to reconstruct a picture of the past.