To me, what separates one hatter from another is in the subtle details of their product. Factory hats will be tougher to tell apart since consolidation & mass production only allows for minor subtle differences to show up. When the old factories were around, the differences were more prevalent but no more.
The custom hatters develop their craft over time & their "signatures" come out. They will be in felt finish, ribbon/bow treatments, & just plain "style". Like that old quality statement, I know it when I see it.