When determining whether a Japanese sword is authentic, a buyer must first decide which of the two definitions of authentic he desires. The most stringent determining factor of Japanese sword authenticity is whether it was crafted by a master of one of the Japanese schools of swordsmithing, and if it is made of real Japanese iron-sand. The second factor of authenticity for a Japanese sword is a stylistic authenticity, meaning that the sword is hand forged using the same folding and two density steel techniques as its ancient counterparts. Using the less strict definition of authenticity, a Japanese style sword can be from anywhere in the world and made of quality steel of any origin. Machine-milled blades lack the metallurgical properties of a true katana, and are therefore inauthentic in the eyes of collectors.
Photograph the hamon, a rippled line that extends in from the blade's sharpened edge. The hamon is significant because it indicates a change in the metallic density of the sword, with the edge being hardened in relation to other areas of the swords steel due to heat treatment. The same heat treatment that creates the hamon also causes the blade of an authentic Japanese sword to take on a slight curvature, giving it the signature katana shape. A straight blade is an indication of a machine milled sword, though such a sword might have a false hamon etched with acid. E-mail the photograph in the highest possible resolution to a relevant antique expert for a professional opinion.
Photograph the maker's mark if one is present. Check the photo against marks illustrated in sword collecting handbooks. If there is no mention of the maker in any of the available antique sword valuation handbooks, the smith may be obscure and require extra effort to verify in a historical context. In order to properly place a difficult piece, consider contracting a Japanese antique expert to certify a provenance.
Check the style of the blade's tang by looking under the handle. Checking under the handle may damage a sword if done incorrectly and should be done by the skilled hands of an antique conservator. A tang is the metal of the blade that the handle is built on. A full tang, as is authentic in Japanese style sword-making, runs though the full length of the handle to create the strongest possible sword.
File a sample of steel from the sword from the tang area to avoid damaging the viable blade. Place the sample in a sealed envelope and send it to a gas chromatography or radiation spectrography lab. Query the lab first to ensure that they will complete the testing as well as to obtain a price quote.
An authentic Japanese sword almost always has a sword maker's mark because swords in Japan are spiritually revered by some. Swords in Japan are considered a matter of high craftsmanship, meaning that a smith will almost invariably want to take credit for their work. Maker's marks are generally written in Japanese kanji and will relate to the smith's name and perhaps the school of smithing they were trained in. Some authentic katanas actually crafted in Japan may be missing a maker's mark from the tang of the blade, which was sometimes shortened along with the handle to meet World War II military sizing specifications. Both private labs and better equipped university labs should be able to conduct the material analysis, though most will charge a fee. Before taking a filing, query the lab to see if a less intrusive form of material testing is available; alternatives vary between labs based on equipment capabilities.
Taking filings from a sword for materials testing may hurt the item's value. Use great care to preserve the aesthetic quality of the blade and its balance by filing only a tiny amount from the bottom of the blade.