You probably won't put junior through college with grandpa's old stamp collection that's up in the attic. Collections put together by amateur philatelists purely for enjoyment are not likely to be worth a lot of money, unless there happens to be a lucky error, or unless the collector invested in some rarities. To find the value of those stamps, whether they be commemorative, definitive, full sheets or blocks of four, you'll need to spend some time with your nose in a book, unless you wish to invest in valuation software.
Obtain a United States stamps price guide. If you are researching U.S. commemorative stamps, many guides can help you. The U.S. Postal Service and Scott Publishing put out annually updated price guides and checklists to help collectors track and value what they have. These guides cost from $8 to $20 as of 2010, although if you just want to look up one commemorative stamp, you could go online to Mystic Stamp Company's online catalog.
Examine the stamp you wish to value. What is its denomination? Commemorative stamps are released at the standard first-class mailing rate in a given year, so the denomination will give you direction to start looking up the stamp in the price guide. Before 1970, when the Postal Service became an independent agency that no longer receives tax subsidies, the cost of stamps didn't rise that often, so you'll have more years to look through in the price guide to find the stamp you're researching. For example, 3-cent commemorative stamps were released from 1932 to 1958. Since the 1970s, the first-class mailing rate has gone up every few years, so anything coming in at 6 cents or higher should be easier to find.
Look at the front and back of the stamp. Has it been used on an envelope and canceled, or is it unused, with adhesive still on the back? If it is canceled, you will find the stamp's value in the "used" column of the price guide. If it is unused and still has its original gum, look for its value in the "unused" or "mint" column in the price guide. Old mint stamps may show evidence of having been hinged and put into an album. The price guide value may specify "mint, never hinged" or the abbreviation "MNH." In general, mint stamps will be worth more than used copies.
Rate the condition of the stamp. Is the design nicely centered? This is "fine" condition and will get you the best price, closest to book value. Are there any rips, creases or other flaws? Is there evidence of a hinge being placed on the back or any thinning of the paper? Does the stamp appear to have been damaged and then repaired? Flaws of any kind, even if repaired, will lower the value of any given stamp and may make worthless any but the rarest of commemorative stamps.
Invest in stamp software that will keep track of the collection for you if you wish to value many stamps for insurance purposes. Annual updates will take care of new releases and changes in value. Software companies release valuation software for countries of the world as well as the United States.
If the stamp is a foreign commemorative, seek out a copy of a Scott Standard Postage Stamp Catalogue, which is released annually in six volumes. The reference section of a library may have a relatively recent copy if you do not wish to put down the $200 to $400 the entire catalog will cost. Often a copy a couple of years out of date will serve a general valuation purpose because stamp values don't fluctuate much from one year to the next, not even rarities.
If you come across an old Minkus price guide (no longer published), the commemoratives and definitives are located in different sections of the book before being arranged chronologically. Other price guides just go chronologically and separate out only special types of stamps, such as air mail, postage due and the like.
Price guides will list stamp values in unused/mint condition and used condition, values of blocks and sheets for each commemorative, and may have notes and values on any potential error versions that leaked out to the public.
If you believe you've found a rarity, fake or error pressing, consult with a dealer or an appraiser at a stamp show or through the American Philatelic Society.
Price guide values are retail values, not what you would expect to receive if you would sell to a dealer, who would pay you wholesale.