Most diesel fuel flouresces to some degree. Use one of those little UV LED flashlights in subdued lighting and you may be able to track down the leak.
If your fuel doesn't glow in the dark or if you want to check the whole system quickly, add a couple of ounces of tracer dye to your tanks, and a different color dye to the coolant system. It doesn't take much to pinpoint even the smallest leak.
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