Have patient hum. If sound lateralizes to ear with impaired hearing then a conductive loss is likely. If, however, sound lateralizes to ear with normal hearing, then the impaired ear may have a sensorineural problem.
Acute onset sensorineural hearing impairment warrants emergent ENT evaluation. Consider starting corticosteroids empirically if there is no clear etiology ie acoustic neuroma, stroke.
Source
Schreiber, B. et al. "Sudden sensorineural hearing loss" Lancet 2010; 375: 1203-11.
Weber, P. "Sudden sensorineural hearing loss" Up to Date. 17 Nov 2009.