There are a few telltale signs that your upright bike belt needs to be replaced. The most common sign is a slipping sensation when you pedal. This can be caused by a worn or loose belt that is not providing enough grip to transfer power efficiently. You may also hear a grinding or squeaking noise when pedaling, which can indicate that the belt is rubbing against the frame or other parts. Finally, if you notice visible cracks or fraying on the belt, it is time to replace it immediately.