If you want the biggest repository of information on Japanese swords I know of, including a great deal on the markings on the tang, google for "japanese sword index."
Couple of points on care: first, never touch the blade with your hands, as the steel is pretty high carbon, and your natural finger oils will eventually etch into the metal. Second, never, ever try to sharpen it yourself; if the blade is actually handmade and not factory (look for an armorer's mark on the tang), the final polishing was done by a second master after the blade was forged by a first master, each a specialist. They used varying thicknesses of rice paper as the only abrasive, with the final one being considerably thinner and less abrasive than a Kleenex (!) And third, if you oil it, make sure the lubricant is totally acid free; carbon again. The traditional care given to a prized sword was to clean it once every two weeks with clove oil. In the interim, if it wasn't needed for anything sanguine, it would be hung unsheathed on two pegs in a place of honor. Oh, one other thing: it's probably not a good idea to keep it in its scabbard, especially it it's made of wood, as it will eventually cause very noticible brownish discoloration of the steel, and nothing you dare put on the blade will remove it.
Actually, once you read around on the site I mentioned, you'll pick up enough info to tell whether the blade was a wartime factory job or a handmade one. There were still a few masters making swords during the war. One possible indicator of its age is the copper band around the first inch or so of the blade, right after the hilt. That was usually only found on wartime blades, although it turned up (rarely) during the Meijei Restoration. As for seeking out a Japanese community, make sure it isn't in Japan; if the blade was made during the period 1929 to 1945 (I think it was 1929, anyway), it's illegal to even possess it in Japan. And "Kill Bill" notwithstanding, it's also illegal to carry any katana openly. They're all licensed, and to go on the street with one, you have to carry it in a special locked case with its serial number tag affixed to the outside; any police or military official can demand to see the tag and force you to open the case so he can verify its authenticity. And Japanese jails are anything but pleasant.
My father brought a daikatana (one longer than roughly two feet, as the prefix means something like "great" or "long") from Iwo Jima, but he came by it the easy way. He was a coxswain's mate on a landing craft, and one of the Marines from his boat came back down to the beach and asked him to hold it for him in the coxswain's tiny cockpit. Unfortunately for the Marine, he never came back. It was mint, and likely an officer's sword made in 1944, but, I think, by hand, as it has all the telltale indications in terms of hardening of the edge, a billow pattern of the secondary hardening, mica flecks on either side of the edge of the billow, and a distinct and slightly irregular grain pattern down the length of the sword. Other things too, but I'm talking too much already.
I have what's left of the blade in a place of honor in my work area. My father gave it to my younger brother (after much wheedling) and the first thing he did with it was chop down a stand of two-inch saplings with it. One stroke per tree. Sadly, it damaged the blade severly, totally destroyed the hilt windings, and since he didn't bother to clean the tree sap off of it, the leading seven inches are edgeless and badly pitted (katana often have small round carbon nodules throughout the steel, and the sap had taken the surface off the tip of the blade and eaten into every nodule it could reach). Then he didn't want it any more and gave it to me. Then claimed I stole it. Much loss of face <g>.
I have minor hopes of restoring it as much as possible, as I, too, regard it as a family heirloom. Oh, incidentally, no handmade katana was totally shiny; any marks left over from the year-long forging and polishing were regarded as part of the character of the blade, and not to be removed, just as the tang was never to be cleaned.
