How do you know when the spark plugs in a 1998 Ford Escort ZX2 need to be replaced?

Poor gas mileage, misfire or the plugs have over 60,000 miles on them. Be careful when you replace the plugs. Don't use the cheap ones, I did that and my 98 ZX2 ran like crap. Put the more expensive ones in like Bosch Platinums or better yet get the Motorcraft Plugs from Auto Zone and you'll be fine... I did this and my gas mileage JUMPED from around 25 MPG up to 36 MPG.... Put a little antisieze on the threads to before installing them, this will make easier the next you change them to get them out.