How do you reduce your risk when you are a passenger in a young drivers vehicle?

Other answer:

First collect information about that young driver,his driving license and quality. If he is above appropriate age, has a state license and knows the roads properly, go with him. As a informed passenger, wear seatbelts and cooperate the driver.
Dimo J:
There is only one way: get out of the vehicle.

If you are training a young driver, keep control of them.

Steal the spark plug wired.
there is no way of knowing unless you know how they drive. when you are with them in the car, do they talk or text on the phone or drive impaired and do they speed without keeping a proper distance? do they take their eyes off the road to talk with friends. do they concentrate? do they analyze dangerous situations. how do they turn left on green lights? do they check blind spot while turning?
If they start to drive the least bit dangerously have them stop the car and get out of it.
100% surefire way, take the keys out of the ignition.