because every single civilization at that time only practiced polytheism according to quite a few texts from varied places.

And its quite "easy". Once again dating things found in the same layer of ground as the people/person, carbon dating it, and comparing evidence to known evidence.

For instance, they can tell celts practiced pagan beliefs, because of written history by the romans, andarcheological finds that are similar to other polytheistic civs. Observation helps here. Of course some are more speculitive then others, but if theres one type pf archeology that has been heavily practiced, its biblical archeology.

It also helps to have an understanding of the differences betwen monotheism and polytheism. Looking beyond your personal comfert zone and knowledge is the best way forward in ihistory