This paper develops a very general test for "monopoly". Using standard comparative statics analysis, we derive testable restrictions on the firm's reduced form revenue equation which must be satisfied by any profit maximizing firm whose choices are not affected by either strategic interactions or the threat of entry. For such an unfettered monopolist, the sum of the factor price elasticities of the reduced form revenue equation must be nonpositive. The set of interesting alternative hypotheses is not empty. We develop simple models of oligopolistic, competitive and monopolistically competitive markets for which this test statistic may take on positive values.