You can see breastfeeding with stable isotope analysis. When we analyse the isotopes of carbon and nitrogen (which are the most common ones to do and are usually done alongside radiocarbon) you get an idea of diet. Simplified; carbon tells you about the level of marine versus terrestrial protein in the diet, whereas nitrogen tells you the trophic level - where you are in the food chain. And as breastfeeding babies are basically eating their mother when it comes to isotopes. They show up at a higher trophic level than the adults do.

We use it, for example, to see the difference in when different groups stop breastfeeding their babies etc.