I've been talking about coffee a lot over the last few years, and despite what I think of myself (an idiot that should never be listened to) I'll do my best to teach you what I think when defining if a coffee is good or bad.
In the most simplistic terms a good coffee is one that tastes good, and a bad coffee is one that tastes bad. Although I have been very clear in saying I'll never tell you what the best coffee is because the profiles are all subjective depending on the person drinking it; there are clear differences between a good coffee and what most people would describe to be a bad coffee.
Let's just get one thing clear right off the bat. I am not even considering instant coffee in this as "bad" coffee. Instant coffee is not coffee and should never be drunk PERIOD. Make yourself a cup of coffee properly or don't have coffee at all. Now that I've cleared that up let's start defining coffees.
So you may be asking what defines a good coffee? Let me tell you.
A "good" coffee starts with a good roaster. Every bean has a different flavor profile to be extracted, and the roaster knows that and how to properly extract it by means of adjusting pressure, roasting times, temperatures, and multiple other metrics to come out with the perfect coffee. You can have the best green coffee bean in the world, but if you don't have a roaster that can extract the flavors properly then you'll end up with nothing. The bean is important but the roaster is just as important. The easy way to roast is when a roaster roasts every bean with zero variation, meaning no change in temperature, pressure, time, etc. This is what I would define as a "bad" coffee because there is no extraction of flavor profiles and the coffee usually comes out bland or overroasted depending on the bean.
So how can I tell if it's good or bad without meeting the roaster? Well I try the coffee! Sometimes I'll drink a bland coffee, sometimes I'll have a phenomenal coffee, it's a trial and error process. Also I don't just write off a roastery because of one bland coffee, I try multiple different roasts of theirs before making a decision as to whether I'll suggest this coffee to others or not.
So if you ask me about a coffee shop/roastery and I give you options just know that those roasteries have been vetted by me thoroughly!
Hopefully you learned something from reading this. If not then you just wasted a lot of time but I appreciate you reading til the end anyway.