How do you know when the timing belt needs to be changed on a 2000 1.4 petrol opel astra?

You will have no symptoms that it needs to be changed. Also looking at it will not tell you whether it needs to be changed or not as it may look good and be ready to break. For this reason timing belts are required to be changed at a set mileage or years interval. Referred to your owners manual as to the maintenance schedule on the timing belt. It varies with manufacturer from 60,000 miles 110,000 miles.