How do scientists test theories?

They start experimenting.

Models stepping in when theories are too complex to handle. Theories may be too complicated to handle. In such a case a simplified model may be employed that allows for a solution

And what car theory test are you referring to?

Scientist theories are the defenations given by the researcheres who search about some matters and proves them and give it a form of definition

it is an educated guess that can be tested by experiment