Sign #1: You heard a single loud noise in the garage, usually a bang.
Garage door springs are under a great deal of tension. If you hear a loud, banging sound inside the garage, it may be a sign that you need to check on the garage door springs sooner, rather than later. If you hear a loud banging noise while raising or lowering your garage door and the door ceases to operate properly, check on the springs rather than trying to force the garage door up or down.
Sign #2: Your garage door will open only six inches before ceasing to operate.
When you try to raise your garage door, it will only open about six inches before it will no longer move. It appears "stuck." It may get stuck entirely in this position, or it may lower to the ground, but not open the remaining six inches in spite of exterior pressure. If your garage door will open partway, it may have triggered a safety mechanism within the garage door that will stop it from putting increased pressure on the spring, which could cause injury.
Sign #3: The door cables are no longer properly connected.
When a spring breaks under tension, the garage door cables often present the most obvious sign to the homeowner: they may seem to have "broken," leaving them scattered in unfamiliar positions rather than hanging neatly as you're used to. In many cases when the cables are disconnected, there is nothing wrong with the cables. Rather, the spring has broken, making the cables useless until they have a solid spring to connect to.
Sign #4: You can't lift the garage door even with the emergency rope.
Your garage door seemed to stick after opening just a few inches, so you pulled the emergency rope to disconnect it from the system and open it manually instead. If the garage door is still stuck, it's likely the spring is to blame.
Sign #5: The garage door moves jerkily or raises crooked, rather than smoothly.
Your garage door should raise and lower in a smooth, seamless motion. You shouldn't notice that it is uneven in any way or that there are distinct starts and stops in the opening process. If you notice your garage door start to open abnormally, it's a good idea to have it looked at by a professional, because it could be a sign that the spring has broken.
Sign #6: The garage door drops fast when you close it.
Rather than lowering slowly and safely, it has a fast drop at the bottom that leaves you a little nervous about anything caught beneath it. Garage doors are very heavy. When it's functioning properly, the spring bears a great deal of that weight. When your garage door spring suffers damage, on the other hand, the rest of the assembly has to bear that weight, and it may not be able to create the smooth motion you're used to.