There are several diagnostics to observe which will help you avoid buying modern reproductions of obsolete paper money which was issued between 1783 and 1866 in the United States by non-Federal issuers. These diagnostics include: 

The Paper -  Modern reproductions are almost always done on a yellowed Parchment-like paper with brown toning around the edges. Genuine notes were never printed on this stuff.

 The Signatures and other "handwritten" information - On modern reproductions this information is printed in black ink. On  originals, this information was almost always written in by hand, usually in brown ink (although on rare occasion in blue, black, or red ink).

The general appearance of portraits and other illustrations - On modern reproductions everything is rather dark and blurred rather than finely engraved with excellent detail.

The word COPY appears on many post-1973 reproductions, usually down by one of the signatures

A number of lists of serial numbers of modern reproductions can be found on the Internet. Check the serial number of the suspect note against one of these lists. If the serial number is on the list, the note IS a modern reproduction. Type in "serial numbers reproductions" into your favorite search engine and you will find several.