A manifold gauge set has at least one sight glass window for viewing contents of a refrigeration system or an air conditioning system being serviced. The window may be non-planar to achieve an appearance that varies with the presence or absence of liquid. Such a non-planar window may be in the form of a dome, prism or a fresnel lens. The gauge set may have a second light transmissive window to allow light to illuminate system contents being viewed. A light source may be provided to illuminate system contents being viewed. The light source may provide light through the same window that is used for viewing contents. A diffuser may be provided with a second window to achieve an illuminated background for viewing system contents. Similar configurations may be used in standalone sight glasses, vacuum pumps, and recovery machines.