How do you know when a stereo sounds good?

because as we know, there are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns—the ones we don't know we don't know.

Is it clear now?

A stereo system sounds good when the experience of listening is about enjoying the music and not fretting about how the sound compares to some absolute audiophile standard. 

In my experience, listeners can achieve this by choosing their main system components and speakers to achieve the type of sound they most enjoy with the music they most enjoy listening to.  Cables and tweaks will not get you there if the main components and speakers are not aligned toward your preferences.  Once I figured out that tone and dynamics are most important to my listening enjoyment, then putting together a satisfying system became much easier.

🤣

when the music becomes so INVOLVING you forget about the equipment, get lost is the experience

Do you have a suite of measurements that removes all shadow of a doubt that you are getting good sound, sound that you enjoy? 

Yes.