This is something a lot of players that play a pre-release comes to mind, your sitting down and you face what looks like a semi-constructed deck with 6 on color bombs & all the good supporting uncommons.

The pre-release is a fun and casual celebration of a new MTG set but often I hear people speak out their concerns that not everybody is that ‘legit’ with their sealed pool. This problem often occurs at larger pre-releases with not a lot of social control around players. In a store I regularly play pre-releases we also have those individuals and the TO/judges don't know how they can fix this issue without hassling with decklists and making it feel like a more ‘competitive’ scenery.

A simple solution I had was, make a picture of your sealed pool after opening it or just show your fellow magicer at the table what you open (what we already do with competitive rel. events) but without the decklist registration. This gives the people that want to add cards to their sealed pool a idea that they can get caught more easily… or is this to simple in the thought process?

I always mention the ‘don’t add cards to your pool' in my opening announcements.
Especially on the sunday of the prerelease weekend.

Your solution is a nice gesture, but if the players make the pictures themselves, there is still a lot of room to cheat.
If you want to use this technique:
Should all opening be done simultanious and with everybody taking a picture directly after opening (and sorting?) the cards?
If you have a dedicated judge who doesn't play in the event, that judge could take the pictures.
I'm not sure how doing this would impact other parts of the player experience.

The other way of discouraging this behaviour can be to flatten the prize pay-out.
I've been to pre-releases in the past where every player got at least one prize booster.
If a tournament is topheavy on the prizes, it's more ‘rewarding’ to cheat.

Originally posted by Alex de Bruijne:

The other way of discouraging this behaviour can be to flatten the prize pay-out.

Alex brings up a very good way for a TO to discourage cheating. While some players will still do it (there are other motivations, like being able to brag to friends), it does change the risk-reward calculations.

Originally posted by Charly Traarbach:

A simple solution I had was, make a picture of your sealed pool after opening it or just show your fellow magicer at the table what you open (what we already do with competitive rel. events) but without the decklist registration. This gives the people that want to add cards to their sealed pool a idea that they can get caught more easily… or is this to simple in the thought process?

I don't think that doing pool registration (even “spread out all your cards and someone will take a photo”) is in keeping with the spirit of Prerelease - people want to get playing Magic as quickly as possible, not wait on verification procedures. Remember that a lot of the players in your event might be kitchen-table players who already feel like tournaments are too slow and bureaucratic, and Prerelease is your chance to upsell them to Friday Night Magic.

If you think a player might be cheating, ask him or her if you can see his or her pool between rounds. Count the rares and uncommons - less experienced cheaters might not consider that they need to swap things out one-for-one. You might also consider asking if there were players who built near this player and who might remember the cards that he or she opened.

The lack of social control you mention stands out to me. I am imagining everyone completely focused on their own sealed pool and too occupied with that to take notice of the people surrounding them. It's the rhythm that competitive frequent sealed players are accustomed to. When there are enough of those players the more casual or infrequent players can easily see that behaviour and copy it assuming that's how they're supposed to act.

But it isn't. It's the prerelease. So mention that in your announcement. Just highlight that for this event specifically players are encouraged to engage with each other. They're allowed to ask for (some) deck building advice or a card evaluation here and there.

Just letting people know that they can look around the table or ask something from their neighbour might be enough encouragement to get your players to do so.

A way to encourage what Quinten mentions is, when the set is faction-themed like Ixalan, to encoruage players to choose a faction (Dinosaurs, Pirates, Merfolk or Vampires), to seat in one of the corner of the venue with their allied, and to help each other to build their deck.
It's also a great way to introduce new players to the community.

- Emilien

Originally posted by Emilien Wild:

A way to encourage what Quinten mentions is, when the set is faction-themed like Ixalan, to encoruage players to choose a faction (Dinosaurs, Pirates, Merfolk or Vampires), to seat in one of the corner of the venue with their allied, and to help each other to build their deck.
It's also a great way to introduce new players to the community.

- Emilien

It doesn't stop players from augmenting their deck during a smoke break or toilet break, which is one of the easiest ways to perform the cheat. It is unfortunately very easy to hide a few crap uncommons or a terrible rare or mythic and add cards from previous prereleases to the pool.

The more people are involved into your deck building, the harder it becomes, and the less people are tempted to do it.
As usual, it's not about making it impossible to cheat, it's about to provide a risk/reward balance that makes it unappealing, and using your investigation skills on the outliers.

- Emilien