I can't tell without looking at the whole sword, and even then it might be difficult. Typically, you find a lot of cheaper swords made in China, and then they carve a Japanese (or Japanese-sounding) name on it to give it the appearance of authenticity. If you have studied Japanese swords for a while, you can recognize a fake signature when you see it. If you can't read Japanese, it becomes difficult, especially without the sword in hand.
If you are looking for an authentic Japanese sword, its best to buy from a dealer who trades in such things. If you try to buy from ebay or any online auction site, you risk buying a fake. Or, you could get lucky and find a real sword, but it might have some fatal flaws (cracks or blisters in the steel).
One thing to help you separate the fakes from the real swords, is the authentication papers that come from the NBTHK (Nihon Bijutsu Token Hozon Kyokai). These are the yellow-ish papers that are issued by the NBTHK (Society for the Preservation of Art Swords) in Japan, and each one carries a photograph of the sword's tang, and tells you what the sword is, when it was appraised, and has its own anti-counterfeit measures (watermarks and embossing). This group examines the sword, then they will issue an authentication certificate if the sword is legitimate.
Other things to look for are patina on the tang, activities in the steel, swordsmith's name that has some history to it, peg holes that are punched rather than drilled*, etc...
*Note some antique swords can have drilled peg-holes, so this is not always a good identifier of fake swords.