go to any auto parts store and rent one. replace your rad cap with the correct adaptor and pump the system up to 16 pounds of pressure, then start pulling and tugging on all your hoses. If after 20-30 minutes your still have 16 pounds you don't have a leak. Clean newspaper a couple of flashlight and an inspection mirror are your best friends when it comes to this.

 

It could be a valley gasket. When mine was leaking it would run down the pass side of the bellhousing and drip onto the cat.