How do you know it's f you need new spark plugs

I replace spark plugs at around 40,000 to 50,000 thousand miles, although some manuals say need to be replaced at 80,000 miles. You will know if you need new spark plugs if these symptoms appear in the car, slow acceleration, loss of power, poor fuel economy, engine misfires, and difficulty starting the car. If you have not replaced the spark plugs wires, I would get a new set installed as well.