A postage stamp catalog is your best bet. You can also check out eBay and Mystic Stamps online for possible pricing.
Almost all countries have issued square stamps. At some point or another you can find a square. The US has issued a large number of square stamps.
One can find more information about stamps for Greece on Wikipedia. They have a detailed history of Greek stamps and full list including the dates they were issued.
Use a catalog to find values. There are several available online, such as Mystic Stamps. You can also find paper versions at most public libraries.
If they are the non-denominated US stamps, there is an identifier on the USPS web site. For collector values, check an online catalog, such as Mystic Stamps.
As of 2013, there are fifteen different websites where a person can find information that regards to the bottle stamps. Some of the names of the websites are Blockhead Stamps, Vanessas Values, and Simon Says Stamp.
You can find them in the catalogs from dealers or governments. There are also catalogs designed for colletors that normally show the stamps in the order they were issued, including the date.
There are several different stamps that fit that description with different values. Go to this site: http://pe.usps.com/text/qsg300/Q604a.htm and find the picture of the particular stamp you have to see its value.
Scott US#1500 1501 1502 and C86There were four stamps issued in a set in 1973. If there is one stamp on the envelope, it is probably worth about a $1. If one envelope contains all four stamps, it would be worth a bit more.
There are a vast number of United Nations stamps issued for use at the United Nations offices in New York, Geneva and now Vienna. Some of the earlier issues have some value, consult a postage stamp catalog to identify and find the catalog values for them.
This is a very admirable hobby to get into as millions do in the US. You can get some ideas of value at values.hobbizine.com/stamps/index.html.
Online sites that have stamp value guides include Stamp Collecting Resources, Stamps Site UK, Hobbizine, Find Your Stamps Value, Stamp Treasures and Mystic Stamp.
hi You can often find the values of Marvel issued cards in Wizard Magazine, I find that to be the most accurate. Hope this helps