Do car rental know if you take a rental out of state?

Well they do check the mileage, and many cars are now equiped with various geo and location finding technologies. So, I'm pretty sure they will know if they care to check.

Answer

Depends on the contract you've signed when renting a car, you will be asked details to where you're taking the car, or are you going to take it out of state. You will be asked to sign a contract, if you're going to take it out of state and you didn't inform them. maybe you'll fine you or something. it's also against the law because you've signed a contract.