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If your bird has just started plucking you may not be able to tell. Some signs are bald patches and chewed quills on the fallen feathers. Another giveaway is an excessive number of feathers on the cage floor in a short period of time.
Edit;
When I say bald, it's not necessarily that you can see skin. You will see a patch that has many feathers missing. Like this, For context, the bird on the far left was moulting. Third along, obviously the plucker.