How will you know when your spark plugs need to be changed?

When a loss of power is noticed or depending on mileage -- newer vehicles with electronics can go 100,000 between changes -- older vehicles with points etc. go about 10,000 between changes When a loss of power is noticed or depending on mileage -- newer vehicles with electronics can go 100,000 between changes -- older vehicles with points etc. go about 10,000 between changes