That would really depend on the sort of behavior you're looking at, and the period and place. I can give you a couple pre-modern examples from European cultures, since that's what I'm most familiar with.

If you were interested in what was or was not permissible in various ancient and medieval European societies, law codes are often a good place to start. There are published translations of a wide variety of law codes, either complete or in excerpts. They can be surprisingly enlightening as to, for instance, what sorts of speech or behavior were considered so offensive as to merit a fine. For the Middle Ages in Europe, there is both a body of church law (canon law) that applied to all Christians and a body of civil law which varied from place to place (or people to people, sometimes). It's important to remember, however, that no law code perfectly reflects how much real people do or don't disapprove of something. Sometimes we find that, for instance, popes or rulers would issue edict after edict against the same behavior over and over, precisely because people weren't obeying the previous edicts. That often indicates that the people who were supposed to obey those laws (and probably even those who were supposed to enforce them) didn't actually disapprove of the targeted behavior very much. Court records are also very informative, but probably not as accessible to the average person than published law codes. You certainly can find published translations of groups of court records in better libraries, though, and almost certainly online.

Contemporary literature is also often a very helpful (if not necessarily efficient) way of getting a sense of the moral and social values of a particular culture. Often when you read a story or epic from a particular place and time, you will find all sorts of clues to what the people of that culture found acceptable or unacceptable because characters in the work will comment on or react to one another's speech or behavior. Of course, it's not always easy to distinguish one person's opinion from what most people of that time and place would have thought, so that is something you have to keep in mind. Sometimes we can learn very odd and surprising things from literature, like in Lancelot, the Knight of the Cart, by Chrétien de Troyes. There, we learn that for a late-twelfth-century French knight, it was completely demeaning to drive a cart! Often in medieval romance literature of this kind, a hero is placed in a position where he must make a very difficult choice between competing values or obligations (here, love and knightly honor), which can tell the reader quite a bit about what the culture as a whole valued.

Finally, for a time and place about which we have enough information, we can assess one individual's actions and words in the context of his or her peers. The brutal conquest of the Americas by Spain and Portugal is a good example because it (quite rightly) elicits horror in the modern observer. We cannot really say that the conquistadores were just acting the way people of their time acted, because in fact many contemporaries objected very loudly to their behavior. Many missionaries and other churchmen, most famously Bartolomé de las Casas (1484-1566), argued that the inhabitants of the Americas should be treated gently, converted, and integrated into Christian society. Therefore it is entirely fair to say that the conquistadores were brutal murderers, because they were regarded as such by many people of their day. What would probably not be fair would be to fault men like de las Casas for failing to appreciate the unique value of the native cultures as they were, and not arguing that the colonizers should just pack up and go home. I think it would be very difficult to find anyone at that time arguing that position, though possibly if you looked hard enough you could find someone! Essentially, you try to figure out what possible range of opinions or actions actually existed at the time, and then assess where a particular person's opinions or actions fall within that range.