Yeah every time I watch The Good Place I realise that the vast majority of people, if they were put in Michael's torture machine, would realise that something was extremely wrong before going to bed on the first night.
Not because the system is obviously unfair and terrible; that's completely fine. People deal with unfair and terrible systems all the time and there's no reason why going to an afterlife that sucks on a cosmic level means that the afterlife is fake -- maybe the universe just isn't fair. No, anyone who was a fraction less self-absorbed than the four humans would've figured it out by the party.
Because Michael does the little orientation video, where he explains the system. He explains that they, the very BEST people, who did the most good in the world, made it to heaven. And everybody else? "Don't worry about it." He says this to a guy who apparently died after donating his kidneys to a stranger on a bus. He says this to a guy who apparently spent half his life fighting for women's rights and the other half for gay rights in oppressive environments. He says this to a woman who apparently arranged, against the government, a volunteer organisation to personally go out there and clear out minefields around areas frequented by children. And then they all go to a big party...
and they don't give a shit.
At that point it should be blatantly obvious that the good place is fake. Because if you gather a bunch of people together based on the criteria that they selflessly gave their lives to help others, often "bad" people, and you tell them "oh, the vast majority of those people you helped will be tortured forever, don't worry about them, you get to relax in paradise, let's have a party", you would have a riot right there at the orientation. "Good" people, by the criteria Michael proposes, the kind of people who do the things that the actors are claiming they do, would simply not accept that; it goes against everything they are and everything they do. They would do exactly what they did for their whole lives, they would exhibit the priorities and personalities that got them into the good place in the first place.
I'd sit through the presentation thinking "okay, everyone's got their brave faces on, they don't want to attack an angel without preparation, this rebellion will require work, fine," and put on a fake smile, but when they gather at the party and there's not a hint of rebellion, of rage, of righteous fury against heaven? At that point, most people would know immediately that the people around them are lying. It just doesn't make sense for 322 people to change so radically in such a situation.
Fortunately for Michael, he did select four of the most self-absorbed people in the world for his experiment, and they don't pay enough attention to notice.