A mental model is a theory about how something works or how things are related. Like scientific theories, they can be disproven. Counter examples must be carefully considered to see whether the theory should be amended or abandoned.
Mental models are great, but in-order to use them, they must be actionable. String Theory fascinates me, but I have no way to put theories about vibrating subatomic strings to any end.
An actionable mental model provides a framework for making a decision, prediction, or suggesting a course of action.
Sometimes mental models are more gut feeling than something rigorously based on evidence. Sometimes they are guesswork based on logic or pattern recognition (or wishful thinking).
Examine your mental models from time to time with a skeptical eye. Look for disproof (avoid confirmation bias).