You mean, if you get an Arceus in trade, how can you tell if it's hacked?
Ways to tell if a Pokemon is hacked:
-The stats are above the maximum they can be.(refer to the Pokedex for max stats)
-Two or more of the Pokemon's stats are at the max. (Pokemon minimum and maximum values are calculated by including natures. A Pokemon can't have the abosulute max in more than one stat because it can't have two natures.
-It knows an unlearnable move.
It is Shiny. Most Shiny Legendaries are hacked, although it is possible to obtain(most of) them shiny.
- I'll add more if I think of them.