On a 1991 Chevy Cavalier how do you know if you need to change the spark plugs or the spark plug wires?
On modern cars you can pretty much go by mileage unless the engine is not running correctly. If you have never changed the plug wires, then I suggest you do so. These wires are almost 15 years old. Heat and age are killers for plug wires. If you have over 75,000 miles on the plugs I would also change them at the same time. Purchase good quality OEM exact fit plug wires & AC/Delco plugs and change them one at a time to avoid crossing plug wires. Apply a slight amount of anti-seize compound to the threads of each plug and do not over tighten. Plugs today are pre-gapped but I would still check to make sure the gap is correct. Quality plug wires come with silicone already inside the boot so do not put any more on the plug or inside the boot.
When you buy your AC-Delco recommended spark plugs, the person who identified the spark plugs for you at the parts shop can find the gap on the computer he used to identify, and gap them for you. Most cases, the spark plug is already gaped because it is matched with the car, especially if you buy AC-Delco Plugs as the manufacturer suggests.