We use Artifacts, Oral traditions, and Written records.
Artifacts are old things. An old soup tin which says "made in Birmingham" on it means that there was a soup factory in Birmingham which made that kind of soup.
Oral traditions are legends and other stories passed down from generation to generation. "My grandfather once told me that his uncle met Louis Riel and that Riel darned his own socks."
Written records are written accounts of events written as histories, or books written at a particular time, which can give you an idea of the way of life then (think Dickens), or ordinary records. Household accounts are a great way of finding out how much things cost at the time they were made. Government and business records are equally interesting.