Instructions
Load your game and go to the menu.
Select "Pokemon" and choose the Pokemon you suspect is hacked.
Choose "Summary."
Examine where the Pokemon was captured. Look in a guide to see where that Pokemon is available and determine if the place of capture listed is possible.
Look at the Pokemon's stats. Use a stat calculator, such as the one found at Pokemon Marriland to determine if the stats are accurate for a specific Pokemon type.
Look at the date that the Pokemon was captured. Hacked Pokemon will often have "January 1" or even "January 0" as a capture date.