My little sis has a 98 malibu with the 3100. How do I test the coolant level sensor on it? Her coolant light came on, but the level is full.

Remove the sensor from the radiator and have her hold her finger on it while you clean the face of the sensor that contacts the coolant. Then put it back in. If that doesn't work, simply disconnect it. It will default to a no fault condition if disconnected.

Where on the radiator is it located?

Usually the passenger side.