How do you know if a Pokemon is legit?

Question

I traded an apparently japanese Pokemon on gts. It's a thundurus, however, it will not summon Landorus (Yes, I do have a Tornadus) . The Thundurus was caught on route five. How do I know if it's legit?

Comments

Hacked Pokemon can't make it onto the GTS, so everything that appears on the GTS is likely legal. Js.

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While answering this I found the answer to the other question you asked, so I answered it just so you know.
 http://pokemondb.net/pokebase/217156/why-is-there-no-landorus

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Indigo, White had two forms of trading, one of them matched you up with randoms and you had to negotiate what you wanted. This had fewer hack-checks.

Please correct me if I'm wrong, but doesn't White have Thundurus, not Tornadus, as the one you catch? Did you trade to get Tornadus as well? I ask this because you MUST have caught one of the two yourself, you cannot trade to get both or it will not work.

Answer 1

Unless there are obvious things like unlearnable moves, illegal Abilities, impossible stats etc, it is impossible to tell if a Pokémon is hacked.
Even if a Pokémon wasn't obtained legally, as long as it is actually obtainable it is still "legit" and should work for your purposes. There must be some other reason you cannot encounter Landorus, because as long as the Pokémon are Thundurus and Tornadus it should appear. To encounter Landorus one of the two must originate from the players game, so this may be the problem.
If you can provide all the details of the Pokémon, such as stats, encounter level, moves, etc I could tell you the chances of wether it is hacked or not.

Answer 2

Well, there are several ways to check if a Pokemon is hacked, including ability, strength, moves, etc. For example, figure out which one is hacked:

Deoxys @ Leftovers
Ability: Pressure
EVs: 252 HP/ 252 Def/
- Psycho Boost
- Cosmic Power
- Zen Headbutt
- Focus Blast

Shiny!Deoxys @ Leftovers
Ability: Wonder Guard
EVs: 252 HP/ 252 Def/ 252 Atk/ 252 Sp. Atk/ 252 Sp. Def/ 252 Spd
- Judgement
- Shadow Force
- Spacial Rend
- Ultra OP Uber Killer Stab

So as you can see, there are several ways to check if a Pokemon is shiny. Another method could be to check whether it has Pokerus. Usually OP Pokemon might not have Pokerus, while a Pokemon with Pokerus isn't necessarily hacked, it's plausible. There's also the option of checking the summary. If the Deoxys was obtained at Level 1 from an Egg in Unknown Area (Can only be reached by the walk through walls cheat).