There are a number of ways you can do this but it depends on your knowledge of electronics and wiring. A simple magnetic switch at the garage door could be used. Using a magnetic sensor that is OPEN when the two magnets are aligned means no signal will pass through until the door is lifted and the magnets are no longer lined up. At that point your signal would run from through the contact and back to your device.
Essentially you can take a small low power light and run the ground line down to the magnet switch and then back up again to the light. When the door is closed the ground does not make contact and the light does not come on but when you open the door the circuit is closed and the light goes on.
If you were to make something yourself it would be done somewhat like this but there are products you can buy to do this also.
Look at this link:
http://www.smarthome.com/7311gd.html