Open the gearbox filler and see if oil pours out or the level has dropped considerably.

Remove the spark plug and see if oil comes out.

Whip the exhaust off and have a look up it.

Seriously doubt that oil has filled your crankcases from the gearbox. There's some pretty meaty joints to get through...

I'd give it all a good clean off with GUNK or something similar, and see if you can spot the oil tracking from somewhere.