An apparatus for detecting the presence of liquid in a high pressure gas pipeline (4) is described. The apparatus comprises a sight glass (2), providing a window into the inside of the pipeline, and a light sensor (1), for receiving and sensing reflected light from the inside of the pipeline through the sight glass. The apparatus also comprises a processor, for automatically detecting the presence of a liquid based on the sensed reflected light. In this way, automatic detection of the presence of liquid in a gas pipeline can be achieved based on the measurement of reflected light, which can be expected to differ when liquid is present compared with when no liquid is present. No visual inspection by an operator is required—although the data can be stored for later operator use or verification if necessary.