How do I tell what players do from what they say in playtests?

You can run the playtests now, but you've hit the real challenge: people don't tell you the truth, at least not in words. They'll say a level was fine while they were visibly frustrated. They'll say they loved it to be kind. What they say and what they do come apart, and learning to trust the doing over the saying is the whole art. People aren't lying on purpose. They want to be helpful, they forget how they felt, they rationalize afterward. But their actions in the moment don't lie. Where they hesitated, where they gave up, where they leaned in, that's the real feedback. Words are opinions. Behavior is data. When the two conflict, believe the behavior. Next playtest, watch hands and faces more than you listen to comments. Note where they struggled without being told to, where they smiled, where they quit. Then compare that to what they say afterward. The gap between the two is exactly where your best insight lives. Train yourself to see it and you'll never be misled by a polite comment again.