Whether in discussion of specific persons or of broader classes of people.
Is it a worthwhile pursuit to try to compare the physical fitness of historical people to that of modern people?
You might be interested in the Did Socrates Even Lift? thread.
They can get an idea from the diet, written accounts, images painted or sculpted, artifacts (weapons and armor used) and skeletal remains--though most of these fall more under the field of anthropology than history. We know that Romans in the first century AD were about as tall as they are today and they lived almost as long. 500 years later, people in Europe were much shorter and died much earlier.