This is a question often asked by people when coming across swords in military fittings.
For the purposes of this question, we will not focus on earlier swords remounted for the war. That leaves 2 categories, that are not strictly correct, but the names have been appropriated by collectors to differentiate between fully traditionally made swords, and those swords more mass produced, that may have been made by a smith, but are not fully traditionally made out of Japanese Tamahagane steel and water quenched.
These two terms are Gendaito and Showato. Although they actually refer to eras of sword making, for the collecting purposes, we will term the fully handmade and traditionally forged blades as Gendaito, and the rest as Showato.
Gendaito are made from Tamahagane, fully forged and folded and water quenched. Water quenching puts a lot of stress on the blade due to the rapid cooling, and can lead to cracks or other flaws far more easily. That is why during mass production, oil quenching is used, as the slower rate of cooling is less risky. But this also means that many of the traditional "hataraki" or crystalline features in the blade are missing. So Gendaito will show far more activities, nie, nioi, and other features than Showato will.
Showato have many categories, some are very close to fully handmade, but lack one or other of the traditional aspects to make them Gendaito.
Among these categories/manufacture methods are the following: (Written by Kevin Jones - Ryujinswords)

• Mill-steel gendaito. Fully hand forged from mill steel or (more often) 19th century railway tracks made from Swedish steel. Differentially hardened in the traditional manner using water as a quenching agent. Possesses an active hamon and hada.
• Koa-isshin Mantetsu-to. Made from Manchurian steel by a special process. Partly forged, partly engineered, and differentially hardened in the traditional manner using water as a quenching agent. Possesses an active hamon and hada.
• Han-tanren abura yaki-ire-to. Partially forged from mill stock, some folding, differentially hardened using oil. Does have a hamon although it is nowhere near as active as a water-quenched sword, but lacks hada.
• Sunobe abura yaki-ire-to. Drawn down, forged to shape, not folded. Differential hardened using oil, may have a fairly inactive hamon, but no hada.
• Mantetsu-to. Rolled from Manchurian railway tracks. Differential hardening using oil, may have a fairly inactive hamon, but no hada.
• Murata-to. Rolled or drawn, oil hardened but not differentially hardened. Yakiba but no visible hamon and no hada.
• Tai-sabi-ko. Stainless steel, oil-hardened, no grain, no hamon, possible yakiba. Made for the Imperial Japanese Navy to resist salt corrosion.
• Machine made. Serial number in the blade. No forging; stamped out and quenched
  in oil without differential hardening, assuming that they are hardened at all. No hada and no hamon. Some may in fact be plated, and in the worst cases the hamon may be acid etched onto the steel. The classic example is the NCO swords.

How to tell them apart from Gendaito is the big trick, and something that mainly comes from study and looking at both as often as possible.
One of the main ways is to look for slightly darker "spots" in the hamon peaks. These almost look like shadows. Not easy to visualize, but once you start to see them, it becomes easier to spot them. This combined with lack of activities, lack of decent hada and lack of nie...points to a mass produced blade. Hamon are also often very regular with regular spaced peaks.

This pic was supplied by member Chris (Vajo) and shows some of what to look for.