How to tell a good speaker, well this one is VERY EASY , if you can spend more than a couple of hours listening then you have a good speaker, normaly the EARS will let you know pretty quick if it's good or not.
Agreed, long-term listening enjoyment is what you want.
But if you don't have the time to adequately audition multiple speakers for several hours each, here's an alternative to help you decide between your final few front-runners:
Make sure the room isn't overdamped. Turn the volume level up a bit louder than normal and walk out of the room. Listen through the open doorway, with no line-of-sight to the speakers.
From outside the room like that, all you can possibly hear is the reverberant field - something that live instruments inherently get right, but few loudspeakers do. If it sounds like live music is happening back in the room, imo that bodes very well for long-term listening enjoyment.