Postage stamps tend to be worth very little unless they are extremely old or rare for some other season, such as a printing flaw or high face value. This means that the value of a large stamp collection may rest on one or two individual items. Going through a collection and assessing each stamp individually may sound arduous. Luckily, there is no shortage of resources for someone wishing to do this, whether online or at their local library.
Research your stamps at your local library, where you should find either a specific guide to the stamps of your country or a multi-volume general guide, publishers of note being Scott and Stanley Gibbons. Recent guides have copious color illustrations, which will allow you to identify your stamps by sight. These guides will usually give three values for your stamps: used, unused and on a first day cover. In this case, the term “unused” implies mint condition. Guides also assign each stamp a reference number. Make a note of these for the more valuable stamps in your collection for the next step.
Go online and browse the philately categories of Internet auction sites. Narrowing your search to that category, check the values of some individual stamps by typing in their country of origin, their denomination (the price printed on them) and their guidebook reference number. Studying the prices individual stamps achieve on a particular day will provide a realistic and up-to-date sense of their worth.
Type the keywords “philately auction online catalog” into your computer's search engine. This should bring up a list of specialist stamp auctioneers that make their catalogs available online. (See Reference 1) Go to one of these sites, where you should be able to hunt for a particular stamp using a search engine. Once again, these sites will offer you accurate information, but don't expect to find inexpensive stamps listed on their databases.
Use these resources to make a list of the most valuable stamps in your collection (those worth over $10 each) and their individual prices. The value of your collection will essentially come down to these stamps, while the rest might achieve a job lot value at auction of anywhere between $20 and $100, depending on the size of the collection.
As a short cut to assessing collection's value, start with American and British stamps, as these are likely to be the most collectible, then browse through for other stamps which look particularly old.
It may be that there are no individually collectible stamps in your collection. In that case, any value it will have will come from its appeal as a whole, perhaps as a starter collection for a first-time buyer.