1. The Paper. Is the image printed directly on glossy paper? If so, it is a modern reproduction. First editions (and even later smaller editions) are printed on matte paper. In the case of Standard (SE) first editions the paper is thick and heavy. In the case of Subscription (RSE) first editions, it is thinner and glued to a heavy cardboard, but it is still a matte finish, not glossy.
2. The Size. For SE first editions, the full untrimmed page should be about 17" x 24". Full folio images are about 13" x 20", and half-folio images about 10" x 13". The RSE first editions have the same image size, and are mounted on a card of 16 3/4" by 23 3/4". Though the printed images vary up to 1" in any dimension from print to print, any greater deviation from these standard sizes suggests a later edition or a reproduction.
3. Format & Mounting. SE prints have usually been cut from their volumes and show one longer rough or trimmed edge. The other edges are usually, but not always, gilt. The SE half-folios may show the title and text on the bottom half of the untrimmed page, or the printing on the front may have been trimmed off. Even trimmed half-folios should usually have text on the reverse, however. The thinner RSE prints will be trimmed to the image and glued to their mounting cards only for about 1/4 inch in from the edges. The edges of the RSE mounting cards are also often gilt. The unglued center of the image will be loose, and some rippling of the loose paper is not uncommon. An image glued to a card over its entire surface is often a reproduction, and less frequently a remounted RSE.
4. Coloring. The RSE prints were all originally hand water-colored. The SE prints may have only the lithographic ink colors as printed, or may have later added hand water-coloring. Both original and later water-coloring will on close examination usually show some texture or depth, particularly in areas of bright color or fine detail where overpainting has occurred. Check the coloring of any SE print against a reproduction of an RSE from a reputable book on Roberts Prints. If the coloring is "wrong", it may not indicate a reproduction, but rather later inaccurate hand coloring of a genuine SE print. Inaccurate coloring will, however, reduce the value of the print substantially.
5. Pinholes. The SE prints will often show small pinholes at the corners of the image. These are registration holes from the printing process, and are a feature of a genuine SE first edition. They were also present on the RSE prints, but were cut off when those prints were trimmed to the image before mounting. Note that sometimes the holes have "healed", either as the result of cleaning or handling. The presence or absence of pinholes, therefore, is not an absolute indicator of a first editions print. Remember, pinholes can be easily forged.
6. Printing in the Margins. Probably the quickest way to spot reproductions is to examine marginal printing. First edition SE prints should have only the block printed title and the F.G. Moon publisher's mark below the center of the image. ANY other printing indicates a reproduction. (The later, smaller, quarto editions have a different publisher's mark and often a plate number in the margin, but they are so much smaller and of such lesser quality that it is hard to confuse them with first edition prints.) Black script titles, fancy fonts or typestyles, or printing in the left or right lower margins of full folio images indicate a reproduction.
BE CAREFUL - There are reproductions out there on everything from calendars to collector plates to telephone credit cards! Most are obvious, but there are reproductions that are the correct size and are not identified as such. Always ask a vendor for a guarantee that you are buying a true first edition, if that is what you intend to purchase.