1. Check your
gas mileage. Spark plugs that are worn down and corroded by burns and carbon deposits will stifle engine performance efficiency, resulting in poor gas mileage. If your car sucking up more gas than normal, the spark plugs would be one of the first things to look at.
2. Hard starting / rough idling. This can also be attributed to your carb/fuel injectors being dirty, which would fall under a general tune-up. However, spark plugs could also play a factor in this.
3. Time between last spark plug change. It used to be every 30,000 miles or so, longer with other cars. If you don't know how long it has been, pull the plugs to check for excessive wear and/or carbon deposits. Be sure to consult with your owner's manual for plug location and/or any special tools needed to access them.
Not my words... but found from a site
