Dmitry, if you can then can you name your posts identifiably, if would not be difficult, would be?
Regarding the subject. The signs of wearing would be different on different profile types. Particularly it is different for each needle on the contemporary “complex” profile. The only know to me way to assess the wearing of the complex profile is to have a reference point how the given needle looked when it was new. With elliptical or conical needs it is simpler but still you need to see under the scope a “progress” in order to recognize the “result”.
Still generally, a needle while it runs in grooves with time loose its cheeks, inappropriate antiscatting screw one side more and another less. Eventually the bottom of the needle become too “skinny” and the needle “sits” deeper into the groove, scratching with its tip the bottom of the groove. This is already a terminal state of the needle and it should be replaced long before it reached that condition. All other answers are possible to get only via a microscope…Rgs,
"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche