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. Air Mail stamps will be found in the back section of the appropriate country.
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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