They sell dye that shows up with a special light and glasses that you put in oil to see where leaks are. My dad used it with good success on an old 93 night hawk. Takes a few oil chabges for the dye to get out the engine but no biggie.
Buy A/C Pro Certified Auto Pro Oil and Fuel System UV Dye Leak Detection for Cars & Trucks & More, 1 Oz, 374CS, Universal: Industrial & Scientific - Amazon.com ✓ FREE DELIVERY possible on eligible purchases
Buy A/C Pro Certified Auto Pro Oil and Fuel System UV Dye Leak Detection for Cars & Trucks & More, 1 Oz, 374CS, Universal: Industrial & Scientific - Amazon.com ✓ FREE DELIVERY possible on eligible purchases