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hankronA few years ago I got a particular blank backed W card (forget the number, W-5XXX). It was so cheap looking I at first wondered if I had bought a Xerox. After looking at it more closely, I saw that it was genuine but(t) ugly.
For these types of black and white picture cards, especially when the collector has not owned one before (like me and that W-5XXX) or it's an unknown item, the collector can take a handheld microscope and examine the picture printing. These types of early cards were usually photoengravings, which is not difficult to identify and differentiate between a modern reprint or fantasy item. The key is that photoengraving was commercially discontinued a good number of years back.
It also shows that a collector can often make reasonable judgements about age, even if he has not handled the item before or is even familiar with the genre (Hollywood picture premiums, foreign cards, etc).