all of the above information helps - also try and watch your bird as it is preening and see how they are treating their feathers. If it is plucking you will see them clearly pulling on the feathers, they may chew, or linger on one part of the feather as well possibly weakening the shaft.
Normal preening is when the bird works smoothly from the root to the end of the feather without much chewing or pausing, though they do work the feather in such a way with their beak and tongue that it could look like chewing - but this will be consistent as they work along the feather, not on one spot excessively.
You can also look at the lost feathers for signs of abuse. Broken quills, split shafts, chew marks, or just overly tattered (they will be mildly tattered, old feathers that they are). Excessive preening can also cause a loss of colour which looks a lot like stress bars on the feathers. > Moulting feathers become loose in the root and should not usually cause pain, or require much effort on the birds part to get them out.
It is hard to describe what is excessive, but even as a new bird person its hard to miss once they get started. The only problem there is that by the time you get to that point it is hard to stop. :/
If you are asking this question for personal reference i hope your Fid is ok.
