The clearest sign is the door that simply won't lift, or lifts an inch and stops while the opener motor whines and heats up trying to force it. That's the opener doing a job the spring is supposed to do, and it will burn itself out if you keep hitting the button.
A second sign, especially on the paired-spring double doors common in this area's newer builds, is a door that goes up unevenly - one side higher than the other, or a visible twist in the panel as it travels. That usually means one spring in the pair has already gone while the other is still holding, and it's a matter of time before the second follows.
If you hear a loud bang from the garage at some point during the day and the door won't move afterward, that's almost always the spring itself snapping under tension. Don't force the door by hand at that point - a broken torsion spring under load is dangerous to work around, and it's the one repair on this list we'd tell you not to attempt yourself.