Using pre-made assets will impact one of the judging categories - Originality. That means that you might get less score than you would get using original assets. nd using pre-made assets in a creative way might make you some Originality points, but not all
There is not a specific way to know if the assets are pre-made. The devs will have to credit the assets and whatever pre-made things they’re using. If a dev hasn’t given any credit, and someone finds out that the assets are pre-made, they can report it to AngrySmile and he’ll take whatever action necessary(the game won’t be disqualified, I think. But will have to include the credits)
"If a dev hasn’t given any credit, and someone finds out that the assets are pre-made, they can report it..."
All of the pre-made assets I will be using are purchased through the Unreal Engine Marketplace, I own the rights and license to them and will not be giving credits in my game. Just so that's out there. They occasionally do giveaways and I have gotten over 100 of their asset packs for free.