How do I tell what users really do versus what they say?

You have hit the oldest truth in your craft: people cannot always tell you what they do. It is not that they lie. They report the story they believe about themselves, and the story rarely matches the hand on the mouse. So trust behavior over words. When someone says a thing was easy but took them a minute of fumbling, believe the fumbling. Watch where they hesitate, backtrack, sigh. Their actions are the data; their explanations are just commentary. Next test, quiet your urge to ask how they liked it. Give them a real task and go silent. Watch what they actually do before you ask what they think. Separate the two, and you will finally see the design as it really performs.