Answer:
You take it to an expert and ask for a valuation.
The above is true, but a good way to see what the MARKET value is, is to visit auction websites like eBay. Do searches for what you have. You can do an ADVANCED SEARCH and check the box for ended listings. Here you can see auctions that have finished, and if the piece sold and what it sold for.
Also you can Google: "discontinued china paterns". You will find many ways then to find others who know a lot about old china and what it is worth.
I had a few pieces of china from the 70's that I knew wasn't being made any more. I didn't have enough for a whole set, so I decided to either get rid of it, or find out how to get more. By following the above solutions I found out that what I had was fairly rare and worth a lot more than I had bought it for in the 70's. So I found dealers and sellers on eBay who were auctioning what they had. Some pieces I actually bought from sellers in Great Brittain! Now I have a complete and valuable set of my china!