Answer from Shekar Dattatri, Conservation India:
Based on years of experience as a once-upon-a-time bird keeper and pigeon fancier, I have to say that it is virtually impossible to definitively differentiate between a plucked feather and a naturally-shed feather through visual inspection. When a wing or tail feather that is still in its growth stage is plucked from a bird, some blood will be evident within the lower hollow tube of the quill, or central shaft. However, when a mature wing or tail feather is plucked from a live or dead bird, it comes out bloodless. Although it is claimed that all the peacock feathers that are sold are feathers that have been shed naturally, this is highly unlikely. Large numbers of peacocks are killed throughout the country through poisoning and shooting, and it is more than likely that their feathers make up a substantial portion of the huge trade in peacock feathers. To be safe it is best to avoid buying peacock feathers or products made from them and prudent to dissuade others from doing so.