[Jared Russell]

Are you talking about elevation/pitch? Or heading/yaw?

Elevation/pitch is best measured with a sensor like a potentiometer or encoder.

Heading/yaw (of the entire robot) is most easily measured using a gyro (or you can use encoders on either side of the robot to derive angle, but once your wheels slip you are screwed). If your shooter is turreted, a potentiometer or encoder is a great way to measure the relative yaw angle between the shooter and the chassis.