It's not true the only way to check is to ask Nintendo. The only way to 100% check is to ask Nintendo. But by checking the hex, the pokemon in a formula, or in pokesav, you can take notes of flaws. You can see if it's missing important hex data or has pokesav data added. You can see the IVs and PID and such to see if they conform, not to mention the game of origin and country among other things. There are also programs like pAC that can check events to determine legitimacy.