The general rule is that if the projector is giving you good images, then the lamp is working fine. That also means that the best method to look for any sign of degradation in the bulb is to look at the quality of the pictures that it produces. But this will be different for different types of projectors. In the case of an LCD projector, the bulb will either get dimmer, or the picture will turn yellowish as the lamp reaches the end of its lifespan. In the case of a DLP lamp, the telltale sign of a bad lamp is the colour banding in the picture.