Look on e-bay and see if you can find a similar watch and what it is selling for. Alternatively go to a bookshop and browse their for a book on the topic. They’re sure to have one and you don't have to actually buy it.

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Try to look it up on ebay–what things sold for there’s becoming a good barometer of the value of antiques and collectibles–often superior than the traditional sources.
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try going to eBay and looking at the average price for most of them.
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search the web
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Guess.

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