A cable that's about to go usually shows itself first: strands fraying near the bottom bracket, a cable that's slipped off the drum grooves and is winding crooked, or a door that suddenly sits crooked in the opening with one side lower than the other.
If the cable has already snapped, the door won't lift - the opener may still hum and try, but nothing moves, or the door drops fast and uneven if it was partway open. Don't try to force it up by hand. A broken cable on one side means the whole door's weight is unbalanced, and that's how fingers and feet get hurt.
On the older extension-spring setups still around this area, watch for a cable that's come loose from the pulley at the top of the track - that one's a fast fail once it starts. If you're not sure what you're looking at, a photo from a safe distance and a call is the better move than opening the door to check.