Start simple/cheap/easy and work your way up. I love the idea of using paper towels / q-tips to check around the return lines, injector nipples, and even the hard lines/nuts on the tops of the injectors.

I'd also recommend using brake cleaner followed by compressed air to thoroughly clean off all the fuel.

If you have a "black light", diesel fuel will fluoresce and it is easy to see the glow at night, so you can clean off all the old, check with the black light to make sure it is gone, and then start the engine and see where the fuel is coming from.

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC