user has controll over the two pieces that you use to "move" the ball
What I need help with is figuring out what sensors to use
If you want to sense a pinball ball, i.e. a shiny metal ball (like a ball-bearing), how about using beams of infra-red light? The ball could break the beam and trigger a signal on one of the Arduino's digital inputs. You will need IR LEDs (permanently on) and IR phototransistors (connected to the Arduino). You'll find quite a few threads on the forum that go into ways to connect IR and visible light sensors to digital inputs.