Shops will sometimes clean the area then add a dye into the oil (or coolant). After some driving (hours, days, months, it depends on the size of the leak) they’ll inspect the area using a UV lamp. The UV lamp will light up only the dye, nothing else, and this makes finding the leak source much easier. These dye kits are available in some parts stores. Sometimes the part store will rent-out the UV lamp too.
Why would you do that for an oil leak?