Researchers interested in studying early populations from before census data was collected must turn to in order to find clues about those peoples.

I believe the answer is: 
-burial sites
-tax record
-archaeological remains

The burial states would be helpful in providing bones remaining to depict the physical structure of the people that burried there. Tax records is useful to understand the way their economy works, and archaeological remains is useful to identify the cultural ceremony or object that is often used.