How do I verify if a event pokemon was genned

[matternet]

Hello, I'm wanting to go to a pokemon tournament and wanted to use a shiny zacian in my game, but I didn't get the event back when it happened, so I'm going to trade with someone for one

But I really wanted to check if the pokemon was genned or not before going to the tournament. How do I check with 100% certainty that it wasn't genned 

[ICanSnake]

You can't. You can only be 100% certain that something is legitimate if you obtained it yourself, but you can make educated guesses and be less confident on legitimacy for someone else's Pokemon, even if it's someone you trust. You can ask for proof to show how they obtained it (a picture of their wondercard in the game with their username, or a video showing the process of redeeming) since you can't fake the card album on the Switch. Depending on what genned means to you, this doesn't prove that something wasn't cloned. Many traders trade cloned proofed events. If you are wanting uncloned, the best option would be to ask for the trading history of that specific Pokemon.