How did the builders of Stonehenge know that this particular type of stone has those properties? They would have had to observe it, in considerable detail, over decades if not centuries to notice resistance to weather erosion. That would require either quantitative measurements and recording of same, or another method such as casting clay molds and making comparisons.

But why would they do the latter, unless they already had strong reason to suspect such durability?

Perhaps they already knew it resisted crumbling and flaking from experience with smaller pieces, and did not care about erosion. That may have been enough for them.