How do you know if it's your Thermostat or your Intake Manifold Gaskets ?

What is your question?

Was reading up on Intake Manifold Gaskets and not sure how to tell if it is a bad thermostat or intake manifold gasket that could be causing my van to overheat all the time. Does anybody know of any very different signs to tell which one would be the problem ??? don't want to replace one to find out it's something else.....