How are smell and taste in fish differentiated?
Short answer
One way to distinguish between the olfactory and gustatory system in fish is by their  anatomical differences. For example, olfactory receptors are clustered in the nasal region, while gustatory receptors are scattered in the head region and beyond. In turn, olfactory information is sent via a single cranial nerve to the brain, while the gustatory system includes three cranial nerves.

Background
The olfactory and gustatory system both receive their inputs through chemoreceptor cells. The chemoreceptor proteins in both systems are G-protein coupled receptor systems. However, the olfactory and gustatory systems are anatomically separated. 

The olfactory end organ is in fish comparable to that in humans, namely sensory epithelia situated in nostrils. The olfactory receptor cells form axons that are grouped into the olfactory nerve (cranial nerve I). The nerve innervates the olfactory bulb, where its axons synapse onto mitral cells. Mitral cell axons group into the olfactory tract that projects to the telencephalon in the brain.  


Schematic of olfactory system of a fish (not all structures shown). Source: Global Warming Australia.

The gustatory end organs are less well anatomically localized. The taste buds can be found in various structures including gills, barbels, fins, oral cavity and pharynx, dependent on the species. Unlike the olfactory system, gustatory information is transmitted to the brain via three separate cranial nerves: facial (cranial nerve VII), glossopharyngeal (cranial nerve IX) and vagal nerve (cranial nerve x). Normally, the facial nerve innervates taste buds on the extraoral surface, the glossopharyngeal on the anterior part of the oral cavity, and the vagal nerve on the pharynx. The facial and vagal nerves respectively terminate in the facial and vagal lobes in the medulla. The glossopharyngeal nerve terminates in a dorsal medullary region between the facial and vagal lobes.
 
Reference
- Hara, Rev Fish Biol Fisheries (1994); 4: 1-35