How do I know if this part needs replacing

This flex disc, sometimes called a giubo, is a flexible coupling used to transmit power from the gearbox to the propeller shaft. Being made of a flexible rubber, the flex disc is designed to allow for some angular movement while reducing driveline vibrations Since this part is made of rubber and is constantly exposed to the elements, it will eventually need replaced in order to continue functioning properly. It is highly recommended that you inspect the flex discs regularly, such as during oil changes. If the flex discs in your car are showing any signs of wear such as cracking or rust around the bolt holes, replace them immediately. A failed flex disc will not only leave you waiting on a tow truck, but also has the potential to damage your car depending on how fast you are going when the flex joint fails as it allows the propeller shaft to strike anything in its reach with tremendous force.