Am I Covered On My Girlfriend's Auto Insurance Policy? 
The most important thing as expressed in the other answers is that you should be a named insured on any policy where you are going to drive the car and aware that you are going to drive the car . Anything short of that is trying to get by without insuring what you are doing and that usually winds up costing more than going with the rules of the insurance companies . Trying to save money at times can cost more than doing it right in the beginning . 