spotter
n.— «One of the greatest enforcers of virtue—a “spotter” in the parlance of the day—was the Rev. Henry N. Pringle, secretary of the Maine Christian Civic League. He appeared in the Queen City on Monday, June 24, to swear out a warrant against several proprietors of business establishments harboring slots. Then, accompanied by an intrepid band of sheriff’s deputies, namely Emerson, Spratt, Powers and Friend, the Waterville minister set out to crush these dens of iniquity where a man could deposit a coin in the hopes of winning a cigar.» —“Bangor has a devilish history when it comes to slots” by Wayne Reilly Bangor Daily News (Maine) June 25, 2007. (source: Double-Tongued Dictionary)