They do not routinely check, however on the rare occasions when it is checked, the consequences are severe: refused admission (so the airline bringing you gets to take you back) and ineligible for admission to Japan for several years. Targetting is partly random, and partly profiled. If you are targeted, all your papers must be in order, and close doesn't count. Any forged documents, such as invalid ticket numbers, will be detected and you probably won't ever be going to Japan again. The US and most countries have similar procedures, it isn't unique to Japan. As mentioned, any ticket out of Japan solves the problem (even tickets to countries where you would not be admitted, ironically enough).