If your spark plugs are going bad then the engine will likely run rough at idle, be hard to start, have poor acceleration and you will likely see a decrease in gas mileage.
Good spark plugs are essential to a properly running engine. I would pull one spark plug out and check for any signs up fouling or a large gap in the electrode and the tip of the spark plug.
If in doubt just replace all the spark plugs with new ones as doing so will increase your gas mileage and improve your engine performance.