Good news is the injector cups or sleeves you're reading about don't exist on a D24 type engine. On a D24 the injectors screw into the head from the outside, with external supply and return plumbing and no internal fuel system plumbing, so any fuel leaks that may occur will leak fuel outside the engine rather than inside it.
Many engines with "unit injector" or HEUI or common rail injection systems instead use internal fuel galleries in the cylinder head for injector fuel supply and/or return. On some of those designs, the injector bore and fuel galleries are separated from the coolant passages by a brass cup or sleeve that is pressed into the head. If that cup cracks or loosens up, it allows pressurized fuel to leak into the cooling system and contaminate the coolant. 7.3L Powerstroke engines are the best known offenders with this issue. That may be the concern you have read about but luckily it is an issue your engine is not vulnerable to.
See ngoma's notes about tracing fuel leaks. Another note -- fuel oil spreads easily and far when it is even a small leak, and airflow from the radiator fan can blow it backwards and even upwards making it hard to find the original source. Seeping rubber injector return lines are a prevalent culprit though.