In order for your garage door to function properly, you need the door to be properly balanced. When it is unbalanced, it will cause additional strain on your opener and potentially cause damage to rollers, tracks, and springs.
There are a few ways to know if it is unbalanced. When it is opening or closing, look at the door. You may be able to see that it is not straight. Another thing to consider is how long it is taking to open or close. If it seems to be slowing down, then it could be due to being unbalanced.
An unbalanced door often makes more noise when it is operating. This is due to the weight of the door straining the opener.