You compare different lineages and use standard linguistic techniques for detecting language change. It’s not perfect, but with long texts you end up doing the

You compare different lineages and use standard linguistic techniques for detecting language change. It’s not perfect, but with long texts you end up doing the same thing because it’s rare for written things to last more than a couple of hundred years without extremely good luck and often it takes just that long for a text to become popular enough for there to be enough copies for you to find and compare.