As was suggested elsewhere, look for dirt/oil accumulations around connections.
Seeing as this is an older Ford, I'd almost be willing to bet just about every spring lock- hose coupling is leaking (o-rings) I used to make a fortune re-sealing Ford AC couplings. Econoline vans where the best, got about 5-6 hrs+ for leak test, reclaim, reseal every fitting & re-charge. It was always the o-rings on the spring lock connections. Almost never saw a compressor or evaporator on a Ford.
If it was a GM, it was almost always the front compressor shaft seal.
If it was a Chrysler, it was an evaporator. Chrysler couldn't source a good evaporator manufacturer to save their ars. Even the new replacement evaporators from the parts dept leaked.
You can usually get a complete seal kit from Ford that has every O-ring for every coupler. You just need the spring lock disconnect tools. LINKY
You can probably get them from Harbor Freight as well.
You might try the local auto parts store and see if they rent an electronic leak detector. Save your coin for the repair parts & re-charge.
Clean the fittings first with some brake clean & compressed air before you use the spring release tool. Makes life easier. Pick up a small bottle of PAG oil to lube the new O-rings and hose couplings on install.