The classic sign is a door that won't budge, or one that slams down the last foot instead of settling. On a house built in the mid-80s range, the torsion spring is usually original or a direct swap from years back, and after roughly 10,000 cycles - which a daily-use door hits faster than most owners think - the coil fatigues and snaps, usually with a loud bang from the garage.
A less obvious version: the door lifts but the opener strains, chain or belt skipping slightly at the start of travel. That's a spring that's weakening but hasn't broken yet - still under tension, just not carrying its share of the door's weight anymore. On a two-car door with a single center spring, that's often a sign the system was undersized for the door it was matched to from the start, which is common in this build era when doors got upgraded but hardware didn't.