I am not sure how to tell the difference between diesel and oil when viewed in the bilge and would like to know the difference for future reference.
Know that it takes very little diesel/oil to leave a sheen in your bilge. A few drops will do it.
Probably the only way to isolate the probem is to eliminate each potential source and then observe. Use a rag and wipe it over all areas of your engine. Leaks will migrate to the lowest point so just because you find one at a low spot it does not mean it comes from nearby. On an old boat, oil and fuel may have accumulated all over the engine from oil changes and fuel filter changes. You really need to clean the engine and the adjacent area before you will locate the problem.
Why not pressure wash? A diesel should not be effected. I am not 100% about this so check with a mechanic first.