[Panace50]

The way any linguist or scientist does...by comparing to archaelogical and parallel societal remains.

The things that are left in the archaeological record are amazing when you look in the right places. Recently we are looking underwater, which often is the right place to check for human remains, as riverbanks and ocean fronts change even in historical times. But when you get serious about checking religious beliefs, they seem to have a very similar central core of beliefs and then a bunch of society specific beliefs that get tacked on by people who want to keep power.