Among the finest French cameo glass to grace collectors' shelves are Daum Nancy pieces. Daum glass had a rather serendipitous start; Jean Daum, a notary and financier, took over an existing company in 1878 before it went under financially, some say as payment for his services. Daum Nancy's longevity is a tribute to their changing designs through attention to each generation's tastes and trends. Daum Nancy glassware still endures today and is being produced under the name Cristallerie Daum.
Depending on the time period of manufacture, Daum glass pieces can look very different from each other. In its earliest years, Jean Daum continued producing the utilitarian glassware and watch crystals already being made by the company of which he took ownership. By the 1890s, manufacturing was taken over by Jean Daum's sons, Antonin and Auguste, who began experimenting with art glass. They debuted their cameo glass pieces at the 1893 Chicago World's Fair. It is believed by some that the Daum brothers' designs were influenced by Emile Galle, which is why some Daum pieces are easily mistaken for Galle.
Following Daum's Art Nouveau period came their post-WWI Art Deco style. This period also marked a shift from handcrafted glassware to mass-produced pieces. This is also the period of time when Daum began producing pâte-de-verre, where crushed or powdered glass is fused by being heated in a mold, then the piece is finished in cameo glass style. (The 1970s saw a reintroduction of pâte-de-verre pieces, which were sold as Pâte-de-Verre Nouveau.) The next major style shift came after WWII, when Daum began making high-quality blown clear lead crystal wares, including figurines. Clear glasswares are still being made by Cristallerie Daum today. There are many other styles that appeared throughout the decades, and it's important to have access to a reliable reference source so you can familiarize yourself with them before making an investment.
Understanding the different style periods of Daum glassware can sometimes help you recognize a fake, particularly if the style doesn't match one of the Daum marks for that time period. As with many companies that produced a large number of wares over a long period of time, many different signature styles were used, from incised to gilt to raised. But one thing held true for all of them, these marks typically contain the Cross of Lorraine, and the marks never took center stage. They appeared near the bottom of a piece, underneath it, or were created in a less obvious way in the lower portion of the body, whereas some reproductions and fakes have marks in the upper third or so of the piece where they can clearly be seen, and they may be done in a more prominent color. A good number of fake and repro marks are of the raised variety, with some being even more obvious because they are not the same color as the body of the piece. Another thing to watch for is different styles of the same letter within the same mark (i.e., the "A" in "Daum" written differently than the "A" in "Nancy") or a letter written in a style that Daum never used. That said, forgers have gotten quite good at duplicating marks, so unless the mark is obviously wrong, more aspects of the piece need to be evaluated.
Some fake Daum marks are painted on or etched into unmarked glassware that the faker then tries to pass off as Daum Nancy pieces. If the quality, color or style of the mark itself isn't an instant giveaway, an assessment of the piece itself will be. The shape may not be one that was ever created by Daum, the thickness of the glass may be wrong, the subject matter may be inconsistent with Daum pieces, or the overall artistic quality may be poor. Daum scenes had depth, detail and beauty, whereas some new reproductions copying original Daum Nancy scenes are flat and lack detail; the images may even be somewhat cartoonish in quality.
Much of the authentic old cameo glass with multiple colors achieves these colors by applying two layers of colored glass over a thicker, solid-colored glass body. However, colored areas on reproductions and fakes are created by starting with a clear glass body, which is far cheaper than colored glass, and applying one very thin layer of color outside and another thin layer inside. This cost-cutting method can sometimes be discovered by holding the piece up to a light and looking very closely for tiny breaks in the color, where the clear glass shows through. Two other telltale signs are color streaking or colored glass pieces suspended inside the glass body, since Daum melted glass pieces and fused them together to form the body, so you wouldn't see individual forms. Again, it's important to check with experts or good references as to whether particular Daum pieces were done with solid-colored bodies, with mottling, or in some other fashion.
The interior of a piece can also help to illustrate the difference between authentic cameo glass and modern fakes or repros. Old French cameo glass interiors are smooth and even, but many repros have depressions where the pattern images (flowers, for example) appear. If you run your hand or just a finger inside the piece, you'll be able to feel it dip down into the pattern of these new pieces, whereas there will be no change on the inside of an authentic piece of cameo glass. These pattern depressions may also feel a bit rough due to the effects of a grinding wheel (which is also what causes the occasional loss of color when the grinding goes too deep).
These are some very basic, general guidelines that can start you on your way to identifying authentic Daum Nancy glassware, but with over 100 years of Daum glassware having been and still being produced, there are many other types of authentic art glass and their reproduction counterparts to become educated about.
Photo via Wikimedia Commons by Maksim.

At A Glance
Signs of a reproduction or fake:
1. Poor artistic quality
2. Incorrect glass thickness
3. Internal indentations under pattern
4. Prominent maker's mark
5. Letters of different stylings within the same mark

Reference books, websites and collecting groups are the best ways to keep from being fooled by fakes. Here are a few: “Daum: Masters of French Decorative Glass” by Clotilde Bacri, Daum Nancy Glass Collectors Club (https://www.facebook.com/Daum-Nancy-Glass-Collectors-Club-131339843552811).