If you grind different coffees at the same setting and make espressos, you’ll quickly find out which coffees are less soluble by the shot times. (faster = less soluble) Or you can do cupping bowls.
If a coffee is less soluble it’s almost always less brittle as well. This makes for a “coarser” grind size. Best way to go without a refractometer.
This concept was the basis for my 80:20 Method. Keep extracting more until you reach a point where it’s not getting better. Then take a step back and you’ll be at the optimum extraction.