Start with a couple of spray cans of degreaser like Gunk, and get the inside of the bellhousing as clean as possible, ditto around the back of the engine and valve cover. Hose it off with hot water if possible. Once it's clean, it's much easier to spot the location of a leak. I used to use a product that turned the oil bright red, and it was easy to see what was newly-leaked oil, and what was residue from previous leaks. It's been over 20 years since I used it, and I don't remember the brand name or manufacturer.
Jerry
