When your experience and ears combine to tell you that you're now hearing and playing something extraordinarily better than anything you've touched before.
Those many years ago when I was a piano student, I was fortunate to play far more good pianos than bad ones. Became quite used to several Steinway and Yamaha grands around town, all in good maintained shape, wonderful action and tone. Baldwins I laughed at, old fogey dead wastes of floor space.
Then I played a Bosendorfer. Once.
I remember it like it was yesterday, and actually now it's almost fifty years ago. I was used to very good pianos, but this was something else. But had I not had an experience base in my head, I would not have heard the difference.
I'd say this applies to basses, and really a lot of things in life, although I've never had my world re-aligned ever again as that Bosendorfer did. Without that mental yardstick, it's hard to know where you're at.