Most evidence tends to be pretty clear even in the field of anthropology. You can publish a speculative theory, but more often a paper is going to say "we found these bones with these markers here, {type of dating} indicates age of X with a margin of error of Y"; "technology X was found at Y at a depth of Z, this matches/does not match technology A in aspects B and C, but not P, Q, etc." Ethnologies are a bit more suspect, but you can check who visited when and observed what, and see if the observations are consistent. And as you might imagine, genetic studies tend to be fairly clear cut.

When you make a more general statement about 'human nature', you start to move into frequency counts of observed societies, which mean that your sampling frame is very limited, and much more likely to give you exceptions than rules. Much of what you see in informal writing is broadly extrapolated from comparison with animals and broad assumptions both about the environment and about humans (assumed lack of) ability to adapt without genetic change.

As a shorthand, as in most fields, if a claim is made and a peer reviewed paper is not cited, assume that this is not the proper source for this information.