A method of measuring the presence of oil in turbine bleed air that uses an aerosol sampler/size classifier to do a comparative measurement of the engine bleed air to the ambient air entering the engine on a real time basis by comparing quantity and size distribution of suspended particles of engine inlet air with suspended particles in the bleed air throughout the normal operating schedule of the engine and determining the presence of oil in the bleed air when the quantity and size distribution of suspended particles in the bleed air exceeds the quantity and size distribution of suspended particles in the engine inlet air by a predetermined amount.