Originally Posted by Diesel911

One of they ways to be certain where the leak is is to degrease the area with Brake Cleaner and get a strong Flash Light and have someone start the Engine while you watch where the leak is comming from.
Another possible source of a leak is the base of the return line Nipple or if the Return Line Nipple on the Injector has a nick on it. If it is leaking at the base the upper part of the Injector needs to be replaced. But, if that is done it will need to be Pop-tested as that would change the opening pressure of the Injector.

If it is leaking between the Injector Nozzle Nut and the Upper Body you can try removing the Injector and retorquing the Injector Nozzle Nut; and install it again and see if it leaks.
If that does not help the Precision surfaces inside of the Injector need to be lapped flat again and the Injector nut torqued properly.

In the attached pic the Red Arrows point to the precision surfaces. The one with 2 arrows indictes both top and bottem need to be lapped.