Put three orthogonal accelerometers, which can measure the expected acceleration of your shock, on your board. Honeywell, Analog Devices and Motorola are some of the accelerometer manufacturers.
You can make a simple shock switch. Take a spring, put the wire along the axis of the spring, and may be a mass on the end of the spring. For example, you can solder the spring and the wire vertically on a board. The wire should be stiffer than a spring. The shock will make the spring bend and it will short against the wire.