I am not familiar with the specific technique you are using for filling of neurons but, in principle, fibers of passage are seen as continuous filaments, whereas terminal fields show up as fine clusters of dots, with occasional branching patterns. You should compare what you see with those images obtained by anterograde pathway tracers, e.g. BDA or Phaseolus vulgaris. There are many studies of this kind in the field.  

Dear Justin (Hi András, how are you?)

It is not an easy question or better it has not an easy answer. In the old times, before the use of synapse markers, referees always asked you to prove that there were synapses by means of transmission electron microscopy. This is still a way of doing it, but it is too time demanding, risky (use of dangerous reagents) etc.. 

The morphology correlates with function. Swellings or varicosities in an axon usually indicate the location where the axon establishes synapses, and there are thousands of examples in the literature. If this is not always true, I'm afraid it has not been published (negative results matter, but they are difficult to publish). If fibres cross the sections tangentially, it is more difficult to asses if there are swellings or varicosities, (depending also on the thickness of the section). If, because of this, you cannot ascertain whether fibres are varicose in the region of interest, there is always the possibility of sectioning in a different plane e.g. horizontal or sagittal. 

But, for a clear demonstration, I would use the technique you are proposing: double labelling with a good marker of synapses.

Good luck

Fernando

By classical IHC (using peroxidase for example), you can distinguish between fibers of passage and terminals. These latter are generally presented as varicosities, but this does not mean necessary that they correspond systematically to terminals. Indeed, depending on sectioning plans. Fibers of passage are presented as continued filaments with different sizes. In summary, you don't need specific method nor specific antibodies to visualize them.