Actually, I am no expert on this, but there are many ways they can try and come to an educated guess. Most of the time it is done by taking statistical data from a large number of remains from the same time period in various places and comparing them with a given set of remains. Then they base it on other factors like historical accounts, artistic representations and other archaeological evidence. But mostly it is from statistical averages across various populations in the area historically and patterns of skeletal data relative to the features in modern populations in the area and elsewhere.
