The facial nerve is kinda like the Golden Retriever (sorry for the mixed cat and dog metaphors) of the Cranial Nerves. Easy going to assess, not a lot of sensory, mostly motor, just does its thing and makes people smile, and adds to their happiness by being a big part of why they can taste things. The sensory responsibility is taste in the anterior 2/3 of the tongue. It provides motor output to several muscles of facial expression including the orbicularis oris (around the lips) and buccinators (cheeks) as well as the stylohyoid and posterior belly of the digastric, both of which elevate and pull back the hyoid. It also regulates saliva production in the submandibular and sublingual glands.
It is one of the longest cranial nerves, originating in the brainstem and branching out throughout the face. The facial nerve gets its info from one side of the brain only, so in an upper motor damage situation, the opposite side will be affected. (If lower motor, it will be the same side).
Evaluating the intactness of the facial is pretty easy, if your patient has a facial droop or weakness in puckering or blowing out cheeks on one side, lack of taste, poor oral acceptance, dry mouth (because of the saliva regulation duties) or one side pocketing of food, we know that the facial nerve is involved.
References below for more information!