Bank was released in Japan, but even so, you can't be certain of legitimacy.
You can look very closely to see if it appears legal though. My Kyogre I got off the GTS was apparently hacked for sure.... But my Shiny Darkrai I got in a trade appears to have been legal(does not mean it was not hacked... but if it was it was a good hack).
Keep in mind too that most legends are likely cloned(which most people do not mind anyway). Probably cloned in Gen IV/V then once they have made it to Gen VI people have found ways to clone in the current generation. If you do not want a cloned Pokemon you have to either really trust your trading partner, or wait until the Pokebank is released to the rest of us and transfer over your own legendaries from your copies of the games. For my own collecting purposes I accept clones as valid.
Unfortunetly, the status screen will only tell you what region of the Pokemon world they came from, and not tell you where they were met. Learn what levels the Pokemon you are looking for was able to be caught at (for example: Dialga/Paklia/Giratina were all available at Level 1 in HG/SS). One thing I have seen is a lot of the Johto beasts at Level 30. They were only available at Level 30 via an event, and hence with an event ribbon should not be able to be traded on the GTS. Most likely you are only going to discover that a particular Pokemon was hacked once you have already traded for it and it becomes too late, as Nintendo lets you see so little information via the GTS before commiting to the trade.
My advice is to look up a Bulbapedia or Serebii information about each legendary, you will find which games they were available in, and at what levels. When you have traded for a Legendary, I recommend you take it to the stat check. If all stats are perfect I would be suspect, especially with Gen III or Gen IV caught Pokemon. Hoenn Pokemon that are perfect are "possible" but required someone to leave they GBA/DS on overnight to have a chance to catch, and at that only very few natures were possible. If it says was a Kanto/Hoenn roamer from gen III they HP can be as high as 31, but Attack can be no higher than 7 and all other stats will be 0.
Ultimately, you need to ask yourself how badly do you mind a hacked Legendary? If you are dex filling, and hoarding them for a trophy, then a hack shouldn't worry you too much. if you plan to trade with others and not cheat someone else out of a decent Pokemon, then you probably will want to spend that much more time looking at all things that might make a particular Pokemon a hack. Anything not caught/bred in x/y has the potenyial to be a hack.