basically what randomspot said, although if you get it from a trusted source, it'll most likely be legit as well.
as for checking the legitimacy of a pokemon, pokecheck is your best option, but a properly hacked pokemon is indistinguishable from a legit one.
That's not so good. I hate the idea of getting hacked pokemon, i mean i bet my opinion is shared by other people, but it kind of ruins the game for people who want to get into legit competitive battling/trading.
Go to the trade forum and find Beldum's thread, then post what you're offering. If someone's interested, then they'll let you know.
Also, use the questions thread. It's stickied at the top of General Pokemon Discussion.
This is coming from the message board that let this topic run for almost a thousand posts. At least Smogon locks threads like this because there's no reason for such threads to exist.
Personally, Smogon gets under my skin because they almost seem like they're saying that their way is the only way to play Pokemon. Serebii has a much more casual appeal to it. I'm as competitive as a Pokemon player gets, but I prefer Serebii because Smogon seems to imply that anything beyond what they say is the wrong way to play the game.