For me, there's a minimum threshold for knowing whether I'm drinking "good" coffee. If there's any hint of bitterness, which might be caused from a bean defect, improper roasting, or stale (read old) coffee, we're not in the right place.
Once that minimum bar has been met, I look for something very simple: whether the coffee get sweeter and/or more complex as it cools.
Most folks demand their coffee hot, and this is because taste buds tend to miss bitter notes at higher temperatures. It's also why people describe most cold coffee as "battery acid". But when the coffee is fresh and of good quality, let that cup sit much longer, and try it cold. If it's still delightful, then that's good coffee!