I'd say a combo of smell/taste. I know my pleco knocks into things like he's blind when he's hunting a shrimp pellet, then chomps down the instant it hits his lips. I'd say that's pretty much the norm for bottom feeders.
Other fish seem to use smell/taste to determine if something is food, then sight to actualy target the bite. Top feeders rarely miss on the first strike, so they've got to be using sight to line up thier lips.