Techniques for determining a thermal condition of a pipeline include identifying a pipeline that carries a fluid at a steady-state temperature, where the pipeline includes a tubular conduit that includes a bore that carries the fluid, and a layer of insulation installed over an exterior surface of the tubular conduit; changing the steady-state temperature of the fluid by applying a thermal contrast to the pipeline; based on changing the steady-state temperature, detecting a thermal gradient between the fluid carried in the bore and at least one of the tubular conduit or the layer of insulation at a particular location of the pipeline; and based on the detected thermal gradient, determining a presence of at least one of water or water vapor between the exterior surface of the tubular conduit and the layer of insulation at the particular location of the pipeline.