The first step in the determination of whether a stamp has a particular watermark is to remove any hinges or other foreign particles (please consult an experienced collector about removing hinges and foreign matter, many a stamp has been damaged by the careless removal of such), gently place the stamp in your tongs and hold it against a strong light source face forward at various angles, being careful not to damage the stamp from the heat of the light source. Often the watermark will come into view, particularly if the watermark is in an area that is not inked. Stamps with a large margin or selvage are prime candidates for this. You have probably seen a photo or scan of a block of stamps where the watermark is clearly visible in the selvage. You should also be aware of abnormalities in the stamp itself: thins, creases, tears, lines in the gum, a heavy cancel, heavy ink or lack thereof, and make a note of where on the stamp these abnormalities occur in order that you not confuse them with a watermark when using a technique outlined below.
The importance of viewing the stamp from many angles cannot be over-emphasized. This applies to any method described in this article.
If the watermark does not come into view when held to a strong light, the stamp should be dipped in fluid, preferably face down. Two of the more prominent types of watermark detection fluids are discussed in another section.
To test your ability to find and distinguish watermarks, the perforated 12 Washington Franklins (WFs) provide the greatest opportunity for U.S. collectors. All perforated 12 WFs have a watermark. Some have double-line watermarks and some have single-line watermarks. You should hone your technique until you can identify a watermark on nearly every perf 12 WF you can get your hands on.
We say "nearly", because the single-line USPS watermark found on U.S. stamps can often be extremely difficult to detect, particularly on orange stamps, but also on yellow and light (olive) green stamps and also on heavily cancelled stamps. Further exacerbating the problem, the single-line watermark is smaller than the double-line watermark. It is possible that only tiny parts of the letter(s) on a particular stamp are present, often giving the impression that a watermark does not exist. On the other hand, imperfections in the stamp may create the impression that a watermark exists when one doesnt. To make matters worse, on some of the later stamps the impression of the single-line watermark on the paper stock became increasingly weaker, to the point of appearing slightly fuzzy, making detection even more difficult.
Become familiar with the design, size and layout of the single-line watermark. Familiarity with this drawing will help in gaining perspective. However, there is nothing like the experience of finding the actual watermark. This is why we can not over-emphasize spending the time with your perf 12 WFs to familiarize yourself with the way the single-line watermarks seem to "hide" in the corners of the stamp. Any perf 12 WF that does not seem to have a watermark provides you with a great learning experience, since it MUST have a watermark.
If necessary, the difficult perf 12 WF should be dipped and re-dipped, examined in the tray or held to a strong light source at various angles until the "hidden" watermark manifests itself. Pay particular attention to two critical intervals, one when the fluid is just beginning to be absorbed and the other when the fluid is just about to dry. Watermarks and other imperfections in the stamp paper often miraculously come into view at these critical moments, albeit briefly. Applying the fluid with an eyedropper will aid in prolonging the moment of initial absorption. Keep in mind that on most of the more difficult stamps, there are tiny parts of several letters on the various edges of the stamp and that the USPS lettering can appear reversed and/or upside down as well as "normal" (see the eight possible orientations).
If after several unsuccessful attempts at finding parts of any of the letters "U" "S" or "P" on the perf 12 WF, you may want to try using a colored filter. If you're really serious about this, you may want to try a photographic or scientific supply house to get filters specifically designed to remove light of a specific color. Alternately, we have found that colored cellophane wrapping paper, for example the kind that fruit baskets come in, often does the job. Either way, you will want to use a filter of the complimentary color of the stamp in question. For example, you would want to use a blue filter for the difficult orange stamp, a purple filter for the yellow stamp, and a red to reddish violet filter for the olive green stamp. The filter should be translucent enough to let plenty of light through and yet block the color of light reflected from the ink of the stamp.
Alternately, you may want to experiment with the color of the tray the stamp is placed in, again using the complimentary color of the stamp for the tray. We have found that the less glossy the tray, the better the results. Thus, opaque glass and ceramic trays often do a better job than the shiny plastic ones.
You may also want to experiment with the light source. Fluorescent light is particularly poor at displaying objects in the red range. This is why indoor photographs often have a greenish tint. Natural sunlight has the greatest range but the sun isn't always convenient. To get most of the visible wavelengths of the solar spectrum without using natural sunlight, it is possible to purchase an "all wavelength" light bulb in most hardware stores at nominal cost. This "all wavelength" bulb may also prove useful in determining the true color of your stamp.
Other points that may prove helpful:
1. Simply knowing that a particular stamp has a watermark and knowing what that watermark must look like, makes finding the watermark that much easier. Study the printable templates and the actual illustrated blocks on the watermark pages until you are quite familiar with the shape and size of each letter. We would also recommend taking a close look at the example on the single-line watermark page and reading the example below it carefully.
2. Many experienced collectors can sort non-watermarked "First Bureaus", the "Triangles" - US's 246-263, from their double-line watermarked counterparts, US's 264-278 merely by examining the raggedness of the perforations. If the perforations are not ragged there is a good chance the stamp has a watermark. Be suspicious of any Number 260, the 50c orange, lack of ragged perfs are a great indicator that the stamp is the less expensive, watermarked US 275.
3. Some watermarked stamps were issued in certain shades of color only. It is sometimes possible to separate these stamps by color or shade alone. The Number catalog is an excellent reference for these shades and we will add to this as time goes by.
The use of any or all of the above techniques mentioned above may just bring that elusive watermark into view.