You will need to consult the catalogs and do some research. Most libraries will have a copy of Scott's Postage Stamp Catalog of the World. You may want to obtain a specialized China stamp catalog.
First of all, you have to identify the specific stamp or card. That in itself can be a daunting task, but most of the catalogs are well illustrated with lots of information. Once you know what item you have, the catalog will provide a set of prices that can be used to estimate value.
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