So, when you go out to buy agate beads or talhakimts from an African dealer in the US, or in Morocco or elsewhere, how do you know if your beads were made yesterday or 100 years ago?

The general rule is to look at the edges and the way the beads were polished. The older beads are completely handmade. Their edges are crisp and you can sometimes see certain irregularities from the fact that it is made by hand.
The newer beads, which includes beads made in Idar Oberstein since the 1960's, were tumble polished. The edges are rounded and soft.
Once you hold them in you hand side by side you start seeing and feeling the difference.

This pictures shows a side by side that should give you an idea.