I wouldn't say it's the only way. You could certainly gather data from experimental archaeology. True, you couldn't duplicate the lives of early hunter gatherer's precisely since you would have to take certain precautions to protect the well being of the researchers and you would be limited in your choice of locations but you could certainly perform experiments that would tell you how much time was necessary for gathering food, hunting, preparing food, making clothing and tools, building and repairing shelters, etc.