We know what we know about prehistory from a variety of sources. These sources tend to be scientific in nature. Social scientists can take the scientific facts and try to interpret them. This gives us a body of ideas that we think are true but which cannot really be proven.
From science, we can know many things about prehistory. We can use things like mitochondrial DNA, for example, to help us know how many waves of people crossed from Northeast Asia into the Americas during the time when the Americas were being peopled. We can use scientific analysis of human remains to determine what sorts of things people ate in given times and places. These tell us some things about prehistory.
We then take the scientific facts and we try to understand what they might tell us about prehistoric societies. Much of this is informed guess work. For example, we know that the “Iceman” who was found in the Alps in 1991 had tattoos. However, we do not know why he had tattoos. Social scientists can try to explain what his tattoos probably meant, but they cannot really know for sure. Science can also tell us what the man ate and how often he had been sick, but we have to use that as a basis to make guesses about what the society in which he lived was like.
Thus, we know some things about prehistory because of clear scientific evidence and some things from informed conjectures that are based on that evidence.