How to Detect Watermarks
A watermark is a design or a pattern within the paper itself & incorporated into the production of stamps. It usually serves as security against forgery. Watermarks often are visible when a stamp is held up to light. An identical stamp may have separate watermarks, for example the George V set of stamps issued in 1911-22 & again issued in 1926 -1933.
One example from the set of stamps issued. Color, Design & Size are the same, but both issues have different watermarks!
The watermark design of a single star on the left side is of 1913 George V Rs 15 and the multi star watermark design on the right is the 1928 issue.
The Single Star watermarked one issued in 1913 is almost 3 times more than the price of the Multi Star watermarked one issued in 1928. Note: This is not an actual scanned image of a watermark in the stamp but done in a graphic image software. There are various fluids which also aid in detecting difficult watermarks such as lighter fluid, benzene or carbon tetrachloride. There are also branded watermark fluids available from stamp dealers. All of them should be used with utmost care.