Share 'Flight characteristics'

Curious what exactly makes a disc overstable or understable. Is it shape, weight, combination of the two, or something else? Without knowing the disc name (Boss, Surge, etc) and it's flight "rating" (-1.5, -2.0, etc) is there some way to distinguish between an overstable disc and an understable disc simply by visual examination. Occasionally, I get a chance to shift through bins of old discs for…