During the film, we see that Thanos doesn't plan to kill 50% of the people for the first time once he gets all the Infinity Stones. He's done it many times before, on many planets. Specifically, Gamora was from a world that Thanos conquered, and he killed half the people there.
During the events of Infinity War, Thanos tells Gamora that the people who live on her home planet now don't know hunger, and are all really well off because of his actions. He also points out that his own home planet, Titan, didn't listen to his warnings and is now a wasteland.
So it appears to me that Thanos has, on probably more than one occasion, seen what happens if he kills 50% of the population, and what happens if he doesn't. We don't know how many trials he ran before implementing his universe-wide plan, but it's not like he's never done it before.