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.
sampling frame is very limited
Seems that this is key. The question is what kind of sampling is broad enough to support what kind of assertion. I'm not sure if that can always be neatly determined, so you might have two sets of claims on a continuum between well-supported and totally speculative with a muddy stretch in the middle.
One fallacy that I see frequently in works of popular science -- and also here on LessWrong -- is the belief that we have strong evidence of the way things were in pre-history, particularly when one is giving evidence that we can explain various aspects of our culture, psychology, or personal experience because we evolved in a certain way. Moreover, it is held implicit that because we have this 'strong evidence', it must be relevant to the topic at hand. While it is true that the environment did effect our evolution and thus the way we are today, evolution and anthropology of pre-historic societies is emphasized to a much greater extent than rational thought would indicate is appropriate.
As a matter of course, you should remember these points whenever you hear a claim about prehistory:
Results that you should have reasonable levels of confidence in should be framed in generalities, not absolutes. E.g., "The great majority of human cultures that we have observed have distinct and strong religious traditions", and not "humans evolved to have religion". It may be true that we have areas in our brain that evolved not only 'consistent with holding religion', but actually evolved 'specifically for the purpose of experiencing religion'... but it would be very hard to prove this second statement, and anyone who makes it should be highly suspect.
Perhaps more importantly, these statements are almost always a red herring. It may make you feel better that humans evolved to be violent, to fit in with the tribe, to eat meat, to be spiritual, to die at the age of thirty.... But rarely do we see these claims in a context where the stated purpose is to make you feel better. Instead they are couched in language indicating that they are making a normative statement -- that this is the way things in some way should be. (This is specifically the argumentum ad antiquitatem or appeal to tradition, and should not be confused with the historical fallacy, but it is certainly a fallacy).
It is fine to identify, for example, that your fear of flying has a evolutionary basis. However, it is foolish to therefore refuse to fly because it is unnatural, or to undertake gene therapy to correct the fear. Whether or not the explanation is valid, it is not meaningful.
Obviously, this doesn't mean that we shouldn't study evolution or the effects evolution has on behavior. However, any time you hear someone refer to this information in order to support any argument outside the fields of biology or anthropology, you should look carefully at why they are taking the time to distract you from the practical implications of the matter under discussion.