https://marco.org/2009/01/12/one-way-to-tell-if-you-have-really-good-coffee
One way to tell if you have really good coffee: these delicious little oil patches floating on top.
It’s pretty hard to get them normally because most paper filters block the oil from passing through. Usually you need a French press or a really good drip pot with a metal-mesh filter. And the beans need to be pretty freshly roasted to have a chance for oil at all.