best slots site for actually testing games before committing money.

i keep seeing people rave about so-called “free mode” for trying out slots, but it’s never clear to me if it’s actually a real test or just a way for the casino to show you some dressed-up numbers. i want to see what happens with the actual math, not some demo that’s been tweaked. anyone else feel like every demo is just a parade of fake wins? i don’t get why no one cares if the demo mode uses a different rtp than live play.