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Well seeing how I trade a lot in the GTS looking for the right nature of pokemon I cant get twice w.o restarting the game. ( Such as cresselia, gallade, Heatran,etc) But my friend just told me there are hack pokemon in the GTS and you might have one, so I ask how can I tell if I have a hack pokemon ._.''
Is there a symbol that tells it is a hack pk?

* I never a encounter anybody who had a hack pokemon so this would be a reason why im asking, for future reference.

Ive seen a Lv.1 Dialga , Palkia, and Giratina... are those hacks then?

If Dialga knows Metal Burst, the Palkia knows Hydro Pump, and the Giratina knows Shadow Sneak, then they're not hacked.

One way to find out if it is hacked or not is to remove all of its EVs and check its IVs. You can restart the game to get the EVs back, or you can just retrain it yourself if you do not like the spread.

There is A LOT of things to show if it is hacked or not. Like DataMasterSarus said: (continuing list)
1. If the name looks unusual (not foreign language, like there's a huge space.)
2.If the moves that you got it with are moves that cannot possibly be learned by that type.
3. If your pokemon is at an impossible evolution (i.e. Cascoon at Lvl. 9)
4. If the Ability does not relate to the pokemon at all.
5. Illegal EVs/IVs (-Luther Olather) Helpful Link
6. More than usual PP (You can use the PP Up though, but still check.)
7. IF it is an EVed pokemon, check for Effort Ribbon(proves it's 100v;ed) & championship ribbon (I mean really, how else would u level it up to w.e. lv it is?!) - Also, most ribbons require something, like if you have master ribbon for contest, does it have any poffins fed? AND if it has that Classic Ribbon and you got it from GTS... Well you shouldn't have because that ribbons prohibits this but idk if it works for hacks or not.
8. Does it have pokerus? Some eved don't, but that would take a longer time to ev, and u can easily get it from a friend so what's the excuse?
9. OT if its a legend/event is very important the wishmaker is often confused because there isn't enough letters for it. Check Serebii or this pg of GameFAQ I found for events. (Use Ctrl+F to navigate faster in second link.)
10. Check WHERE it was found, some like starters, can only be breed/catch at what ever place you start from.
11. Check what level it was met at. Is that possible? (Check marriland.)
12. The pokeball they were captured in, if it was breed then it has to be a Pokeball/etc
13. The ID # is its event.
14. Correct stats? look up your pokemon if its stats say it was ev in three stats or more then it's probably hacked.
15. Check the contest's status pg thing of your pokemon, where it says the sheen/poffins, if it's all max well... You know.
16. If the person said its a certain Hidden Power and its not, I wouldn't trust the other things the person said but it makes me wonder if it's hacked or not due to the unwanted Hidden Power... Hm...

I'm sure there are other ways to check, but these are some quick to see if so or not.

if it was caught in the mystery zone or lvl 0, if it was caught in an unusual place that you wouldn't normally find wild pokemon (i.e pokemon league)
unusual moves

i belive if you get HGSS pokemon they say "From a Far away place" (i traded with my self and thats what it said)

yup now a days thats nintendos way of preventing hackers from finding out about the plans for the new game so they wont find a code in the program that says "From Jhoto"
(or nintendo was lazy....)

..An Everstone is used to prevent a Pokémon from evolving.. It has nothing to do with making one evolve before it should.

What #3 means is that if you come across something like a Swampert at level 35 or lower. It's obvious that it was hacked, because any true Marshtomp can only evolve into Swampert at level 36.

I had this issue seeing Delcattys online at level 1 or so. x) I'd forgotten that Skittys evolved using a Moon Stone, so level didn't matter!