The Collaborative International Dictionary of English v.0.48:

Inspector \In*spect"or\, n. [L.: cf. F. inspecteur.]
   1. One who inspects, views, or oversees; one to whom the
      supervision of any work is committed; one who makes an
      official view or examination, as a military or civil
      officer; a superintendent; a supervisor; an overseer.
      [1913 Webster]

   2. A police officer, typically holding a rank one below
      superintendent, and in some cases in charge of several
      precincts; as, inspector Clousseau is investigating the
      case.
      [PJC]

   Inspector general (Mil.), a staff officer of an army, whose
      duties are those of inspection, and embrace everything
      relative to organization, recruiting, discharge,
      administration, accountability for money and property,
      instruction, police, and discipline.
      [1913 Webster]

WordNet (r) 3.0 (2006):

inspector
    n 1: a high ranking police officer
    2: an investigator who observes carefully; "the examiner
       searched for clues" [syn: examiner, inspector]

Moby Thesaurus II by Grady Ward, 1.0:

83 Moby Thesaurus words for "inspector":
   Big Brother, G-man, MP, auditor, bailiff, beadle, beagle,
   boatswain, boss, bound bailiff, captain, catchpole, chief,
   chief of police, commissioner, comptroller, constable, controller,
   deputy, deputy sheriff, detective, examinant, examiner, fed,
   federal, flic, floor manager, floorman, floorwalker, foreman,
   gaffer, ganger, gendarme, government man, head, headman,
   investigator, lictor, lieutenant, mace-bearer, marshal, monitor,
   mounted policeman, narc, noncommissioned officer, observer,
   officer, overman, overseer, patrolman, peace officer,
   police captain, police commissioner, police constable,
   police inspector, police matron, police officer, police sergeant,
   policeman, policewoman, portreeve, proctor, reeve, roundsman,
   scrutineer, sergeant, sergeant at arms, sheriff, sirdar,
   slave driver, straw boss, subforeman, super, superintendent,
   supervisor, surveyor, taskmaster, tester, tipstaff, tipstaves,
   trooper, visitator, visitor