A few signs tell you it's the cable and not the opener or the spring. If one side of the door looks lower than the other, or the door hangs crooked in the track, that's usually an uneven cable - one side has slipped off the drum or frayed enough to stretch. A visible kink or a strand of wire poking out near the bottom bracket is a cable on its way out, not a cosmetic issue. If the door won't move at all and you hear a loud snap right before it stopped, that's very likely a broken cable rather than a broken spring, though the two can happen close together since a snapped cable sometimes takes the spring tension down hard. Rust streaking down the cable from the drum is a good early warning sign in this climate - cold, damp winters followed by salt and slush tracked into an unheated garage corrode cable strands from the inside out before you ever see a frayed end on the outside.