I have noticed that ebay has a number of sellers who are knowingly selling reproduction art nouveau style enamelled jewellery as original.  This will give some guidance on how to spot it.

Hallmarks

Most but not all silver jewellery from this period is hallmarked.  If the hallmark is on a raised edge then it may have got worn somewhat, but it should still look crisp in parts.  If enlargements of the hallmarks look grainy within the hallmark, it is possibly a casting of the original piece.  This is unusual though, because casting hallmarks is a very serious offence and most reputable casters won't do it.  It is not an offence to cast makers marks though.  If you see a Charles Horner piece with the CH makers mark but no hallmark then it's a copy.

Some seller's knowing the maker of a copied piece will claim that the hallmarks and maker's marks have been 'rubbed' away, though the enamel is still, miraculously, in perfect condition!  They are right.  The hallmarks and makers marks were rubbed from the mold of the original piece prior to casting.  I'm going to add pictures of the backs of repro jewellery as I get them but I'll start with this one below which is an obvious copy ... no way can makers marks or hallmarks get rubbed from a hollow!  The seller knew it was a copy, sadly the buyer paid in excess of £200 for something that normally sells for £40ish.


ETA:  I have just been contacted by another ebay seller asking me to correct my guide.  The above comments are in relation to the stamped CH maker's marks, not the intertwined CH marks which are cast into the silver. ALL stamped maker's marks should be accompanied by full Chester hallmarks (Horner only used the Chester assay office and not the Birmingham one). The seller also suggested that you can tell reproductions just by looking at them.  Two points.  1) on ebay you cannot handle the product, only go by photographs. 2) items which have been professionally cast are almost impossible to tell from the original.  You have to go by the hallmarks.  I know this because as well as owning original Horner pieces I also have a good few quality reproductions (well the thought of losing a £300 necklace if I wear it out is just too much.  I only wear the real thing on special occasions and if I don't have a copy).  Hope that clears things up.  And yes, I haven't forgotten that I said I'd put  more photos of the original hallmarks and the *rubbed* ones on copies.  Just a bit busy at the mo!