Just how much carbohydrate our ancestors ate is debatable, but 80 g/day is not necessarily non-ketogenic, if the person is insulin-sensitive.
Isotopic analysis of bones is one way. Dr. Michael Eades has a lecture on “Paleopathology and the Origins of the Paleo Diet,” available on YouTube, in which he discusses this.
Again, isotopic analysis of bone remains is a good indicator. A population eating exclusively meat has to have been in ketosis all the time.
Most primitive societies had a traditional diet that involved eating almost exclusively meat. They regarded plant foods as fit only for eating during times of famine. The more isolated of these tribal societies can certainly be taken as suggestive, even though they are not proper “evidence” of what our ancestors ate. Certainly the experience of the Pima Indians, the Inuit, and the Maasai, among many others, where the transition from the traditional diet to the Western diet has been observed, shows that a diet low in sugar, starches, grains, and other carbohydrates is extremely healthy. George Mann in the 1960’s examined around 400 Maasai and could find no evidence of heart disease, except for one man, whose EKG was inconclusive. An earlier report by the British colonial administration compared the health of the Maasai very favourably to that of their vegetarian neighbours, the Kikuyu, who were not nearly as healthy.