Summarize the following:
Stamp catalogs and price guides are excellent resources, having illustrated lists of stamps, arranged by year, that give a current market value for a given stamp issue. The most widely recognized catalogs are: the Scott Postage Stamp Catalog, Stanley Gibbons for Great Britain issues, Yvert et Tellier for French issues, Unitrade for Canadian issues, and Minkus and Harris US/BNA for United States issues. You can often find these books in a large library, if you don't want to purchase them yourself. With the designs of many stamp issues only differing by a line or a dot, magnifying glasses are probably the stamp collector's most valued tool. Small jeweler's loops are effective for most hobbyists, but extra valuable or difficult to identify stamps could call for a high-powered magnifying glass with a built-in colored light source. This tool measures the size of the perforated holes around the edge of the stamp, and is only necessary for advanced stamp collectors. These gauges tell you how many perforations fit in 2 centimeters (0.79 inches), which can greatly affect the price of a valuable stamp. If a stamp guide lists two numbers, such as "Perf 11 x 12," the first number refers to the horizontal perforation and the second number refers to the vertical. The paper used to print stamps sometimes has a watermark, often too faint to identify by holding it up to the light. If you have a stamp that can only be identified with a watermark, you'll need a special watermark detection fluid that is non-toxic and safe for stamps. Place the stamp on a black tray and drip the fluid onto it to reveal the watermark.  This is also a good way to find creases and hidden repairs on a stamp.  If you don't want to get your stamps wet, purchase a specialized tool for this purpose, such as a Sinoscope or Roll-a-Tector.

Summary:
Refer to stamp collector books. Examine stamps with a magnifying glass. Use a perforation gauge. Identify watermarks.