I like to spray things off with brake cleaner and get everthing nice and dry. Afterwhich I let the vehicle idle while I observe with a strong Flash Lignt and run my fingers around the suspected area. You will find where the leak is comming from.
Where they leak with the most likely at the top of the list.

Fuel Injector Return Hoses (There is several threads on where to get it)

Fuel Injection Line Cracked or has a pin hole (mostly on the newer Engines)

Leaking between the 2 halves of the Fuel Injector; where the upper body and the Injetcor Nut that holds the Spray Nozzle together. (most ofte seen right after Injectors have been apart and re-assembled but "ounce in a blue Moon the halves loosen")

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84 300D, 82 Volvo 244Gl Diesel