Garage door openers have come a long way since they were invented in the 1920s. Since 1993, all garage door openers sold in the US must come with safety features such as photoelectric sensors or safety edges which reverse the door's direction when it encounters at least 15 pounds of pressure.
These safety features are only good when they work, however. That's why it's necessary to test the features out every once in a while.
I know that my pressure feature on my garage door works. The wife is always parking the car with the back end too far out, causing the garage door to hit the trunk of the car, reversing and forcing her to start the car again to move the car in a couple more inches. But if you want to test this, take a 2x4, put it on the path of the garage door, and make sure it reverses it's path when it hits the 2x4.
The other test is simple. Take a box that is at least 6 inches tall, and put it in front of the electric eyes. If it starts to operate, you have a problem.
But the most important garage door safety tip is to teach your kids (or grandkids in my case) not to hang out under the garage door when its closing.
Take a quick look at this video for a demonstration of testing safety features on your garage doors. These quick tests can make sure the garage doors in your house are safe and won't injure your kids or pets.