Most garage doors have an electronic beam of light at the very bottom. These devices are intend to prevent a garage door from continuing to close if the beam of light is broken, by perhaps a pet or child that runs through the beam while the door is closing. You should test these beams to ensure they are working properly, as often times the wires get frayed or bent during various cleaning or other events in the garage.
In order to test your garage doors safety features, you will need a box and a small, one inch piece of wood. With the garage door open, place the box in the doorway so that it is blocking the small electronic sensors on both sides. If you try to close the door, the door should not close if operation properly.
Another test is to place a small piece of wood in the doorway that is smaller than the electronic sensors which are typically six inches above the floor. Close the door and watch to make sure the door reverses itself when it hits the piece of wood.
We found a great video from LiftMaster on how to test your garage doors.