If you are offered a "genuine original " bookplate from George Washington's library be very skeptical . Here are some simple rules to follow when examining such plates.Hold the plate up to the light and see if the paper has chain-lines. If it does not then it is not an original plate from Washington's library.In the illustration above a genuine plate has been back lighted and enlarged so that you can see the 18th century paper more clearly.
The copper engraving plate for Washington's exlibris was passed on to descendants and they pulled restrikes for friends .The restrikes are desirable since an original may not come your way for a while. I have scanned a clear image of a restrike .
If you look at it you will see another important distinguishing feature. I inserted a colored arrow pointing to it and I have also circled it. When George Washington's plate was made the engraver's burin slipped and made a small line. If that line is not visible the plate you are being offered is a fake.