I think oil burn BLACK because of the lubricant additives. Wheres, petrol doesn't have any of this hence it burns WHITE... But since I'm no expert, I could be wrong...
Anyhow, there's another way: You can practically SMELL the exhaust while the engine is running... Its not a healthy practice but at least it will tell you exactly what's wrong with the engine...
So, it the smoke smells like petrol, then the carburetor is over flowing... If it smells like a 2-Stroke Rickshaw (LOL!) then you'll have to change the head gasket...
Alternatively, you can take-out the spark plug, and if its 'terminal' (the tip that goes in the cylinder) is EXTREMELY BLACK then its usually associate with head gasket leakage or a worn piston ring... And if the tip is clean then its probably the carburetor problem...
Hope it helps,
Regards,
Mr. Nine.