No single test proves a piece is original. Dealers who have handled posters for decades say the same thing: look at the printing, the paper, the size, the marks and the wear together, and ask whether they all make sense for the date and the kind of piece. Here is what to check.
Look at the printing under a magnifier
Magazines and most posters from the 1970s to the 1990s were printed by offset lithography. Under a 10x loupe, offset colour shows tiny dots of cyan, magenta, yellow and black that sit in small rings called rosettes. So dots are not a warning sign; on a piece from this period they are what you expect. What deserves a closer look is printing that doesn't fit: soft, fuzzy edges on small lettering, which a conservation guide associates with inkjet copies, or the blurred look of a scan of a scan, where fine print turns "mushy". Some modern presses use random-dot screening too, so the dot pattern is a clue, not a verdict.
Feel and look at the paper
Original posters and magazines were commercial products, so the paper varies: thin, glossy, matte or sturdy. Age tends to show as even toning, edge wear and handling marks, and on folded posters wear follows the folds. None of these proves age by itself, and a clean piece is not a fake for being clean. Be wary of a "vintage" magazine page on thick, bright white art paper, which is not what magazines were printed on. On some thin older poster papers the image shows faintly through from the back; modern copies are often on opaque stock.
Measure it
Japanese posters follow the JIS B sizes: a B2 is about 515 x 728 mm, a B3 about 364 x 515 mm, a B1 about 728 x 1030 mm. A "Japanese B2" listed in American sizes such as 18 x 24 or 24 x 36 inches is a warning sign. Magazine pages follow their magazine's own trim, often an "A4 variant", so compare against another page from the same issue rather than against A4. A wrong size isn't the end of the matter, since a piece may have been trimmed or issued in more than one format, but it should prompt questions.
Read the small print
Printer and publisher lines, distributor names, release dates for a film or video, and a 非売品 ("not for sale") mark on shop posters all help match a piece to a known issue. Missing or wrong details, odd spellings and blurred credits deserve a closer look. On their own, though, these marks can be copied.
Turn a magazine page over
A page cut from a magazine is printed on both sides, so the back shows whatever came next in the issue: an article, an advertisement, another picture. A period reverse is a strong sign the page came from the magazine. A blank back calls for more questions. Furoku posters given with magazines were often printed on both sides, so a second picture on the back is normal.
Check how it has been kept
Folds, pinholes and old tape are part of a working poster's life. Linen backing is an accepted conservation treatment, but it hides the reverse, so a backed poster relies more on the seller's knowledge. Dry mounting (gluing to board) is permanent and generally avoided by collectors.
Watch the seller
Red flags include a price far below the going rate, photos that don't match the description, many copies of a supposedly scarce piece, vague answers about where it came from, and reliance on a certificate in place of the object itself. A certificate is only as good as the person who signs it. The safest buy is from a seller who describes condition plainly, photographs the actual piece and stands behind what they sell.
We apply these same checks to every piece we list, and we say what we know and what we don't.