how they use to test whether the ND is good or not? actual picture taken? Must have pic to prove the quality of these filters.
For vari Nd it is very simple. See if got color cast, how much color cast, and at what stop will cross banding appear. And the wider the lens the more propensity tonsee cross banding.
For resin rextangular nd, key is to watch out for color cast, and also evenness of the nd.