A system for detecting the presence of one or more fluids in a rotating component of a gas turbine engine. A first reflector structure includes a first face that receives light from the light source. The first reflector structure reflects at least a substantial portion of the received light from the light source if a second face thereof is in the presence of a first fluid and does not reflect a substantial portion of the received light from the light source if the second face is in the presence of a second fluid. A reflection receiver structure receives light reflected by the first reflector structure. If the reflection receiver structure receives a first predetermined amount of light reflected by the first reflector structure it can be determined that the second face of the first reflector structure is not in the presence of the second fluid.