Anti aliasing after the fact is not really feasible without taking a lot of real detail out as as well. Anti aliasing has to happen during scanning. Later the damage is baked in for good. As far as visibility of this is concerned it's highly correlated to the high frequency content of the images. Where there is none it's hard or impossible to see. When there is a lot the image is riddled with false detail. Non horizontal/vertical hard edges are usually the first place where one notices the problem.