On the Dynamo Air Hockey machines, they use a magnetic burglar alarm switch mounted to the end of the coin slide.
Radio Shack # 54-627 will work.
The magnet mounts on a piece of plastic attached to the end of the coin slide. The actual magnetic switch is mounted in the machine and positioned so the magnet swipes just above the magnetic switch at the end of the slide's stroke when someone puts coins in.
Please note that these kind of magnetic switches can only switch very low current low voltage loads like say to a 12 volt relay coil or logic IC chip.