Here's how you test for looming problems:
Harmonic balancer. Like any other engine internals, it will rap, knock, or tick long before it fails. Rev the engine steady in neutral at 2,000, 3,000 rpm and listen carefully for noises; listen also when you release the pedal. Check for noises at cold startup and at other temperatures besides fully warmed up. Also, have the crankcase oil evaluated by a lab for evidence of high metal content, indicating excessive wear.
Alternator. Like other components driven by the accessory belt, listen for unusual noises at various engine speeds in neutral. Use an automotive stethoscope to help pinpoint faulty parts. With the car off, connect a voltage meter to the battery (should read 12.5-12.65 volts), then start the car (voltage should climb to 13.0-14.5 volts);
stress the alternator by turning on the headlights, the radio and the air conditioning. The voltage should remain high with these circuits on. If the voltage does not change when the engine is started, if it does not get above 13 volts, or if it charges above 15 volts then the alternator may be faulty.
Radiator and water pump. With the engine running and fully warmed up, scan the entire surface of the radiator with an infrared thermometer. If there are distinct cold patches, the radiator may be partially clogged. Using a strong flashlight, look for corrosion, especially along any seams. Also with the flashlight, look around the pulley of the water pump, close to the belt, for any dry coolant stains. Listen for any gurgling sounds while driving or while the car cools in the garage. Check coolant reservoir at cold for any slight loss of fluid over the course of a month. Drive the car from cold to fully warm, noting the temperature gauge. It should rise to 180-190 degrees F. within 15 minutes and maintain that temp. at highway speeds; it should hover around 220 deg. F. in dense, stop-and-go traffic and never exceed 230 deg. F.
An OBD scanner is helpful for preventive diagnostics. It can show trouble codes that are happening and stored in the car's computer, but infrequent enough not to cause a dashboard light.
I've been driving nothing but used cars for 35 years and have never been stranded using these techniques. Most of those cars have been older than ten years. One of my current cars is 27 years old!
By the way, you forgot to mention tires. You'd be amazed how many disasters I've avoided by checking the air pressure and feeling the treads with my hands once a month!
