 It is detectable by the presence of a black ooze around the exhaust manifold, piping and turbocharger, if fitted.It can be mistaken for lubricating oil in some cases, but it consists of the "heavy ends" of the diesel fuel which do not burn when combustion temperature is too low.The heavier, more oily components of diesel fuel contain more stored energy than a comparable quantity of, say, gasoline, but diesel requires an adequate loading of the engine in order to keep combustion temperature high enough to make use of it.