Most people notice one side of the door hanging lower than the other, or the door going up crooked and binding in the track. Sometimes it's a loud bang followed by the door refusing to move - that's usually the cable coming off the drum entirely rather than fraying slowly. On older torsion setups common to houses built here around 1970, you might also hear a scraping sound before full failure, which is the frayed cable catching on the drum groove. If you hear that, stop using the door. A door that's still closing on one working cable can come down unevenly and put stress on the opener or jump the track, which turns a cable job into a bigger one.