An interface probe uses a conductivity sensor or a combination of refractive index/conductivity sensors to determine the presence of air, water, or hydrocarbon within an observation well. The system is calibrated so that the depth from ground surface to the liquid levels can be accurately recorded. An interface probe is a portable electronic system that is simple to operate and requires minimal maintenance. A minimum thickness of 1/32 in. is detectable with an interface probe. The interface probe resembles a common tape measure. It is very useful when used alone with soil gas probes during site investigation. The probe is used in wells to detect the level at which both oil and water are present.