The best way to tell if a cup of black coffee is bad is to conduct a sniff test. When coffee has gone bad, its smell will start to fade.
Bad coffee can taste like a few different things when it has gone bad. Dull and flavorless are the most commonly used terms to describe the taste of old coffee.
When fresh beans are brewed, a thin layer of crema will form on the top. Thin black coffee with no microbubbles is a sign of old beans.