Polygraphing your employees is an issue that comes up from time to time. You may want to incorporate that procedure in your interview process or utilize a lie detector as part of a particular investigation. As you can see from the federal statutes below, the general rule is that an employer may not require a lie detector test. However, if you keep reading you will see that employers in the security alarm business are exempt from the prohibition. Keep in mind that this is federal law and your state may have its own rules. I haven't checked state laws for this issue.
29 USCS § 2002
§ 2002. Prohibitions on lie detector use
Except as provided in sections 7 and 8 [29 USCS §§ 2006, 2007], it shall be unlawful for any employer engaged in or affecting commerce or in the production of goods for commerce--
(1) directly or indirectly, to require, request, suggest, or cause any employee or prospective employee to take or submit to any lie detector test;
(2) to use, accept, refer to, or inquire concerning the results of any lie detector test of any employee or prospective employee;
(3) to discharge, discipline, discriminate against in any manner, or deny employment or promotion to, or threaten to take any such action against--
(A) any employee or prospective employee who refuses, declines, or fails to take or submit to any lie detector test, or
(B) any employee or prospective employee on the basis of the results of any lie detector test; or
(4) to discharge, discipline, discriminate against in any manner, or deny employment or promotion to, or threaten to take any such action against, any employee or prospective employee because--
(A) such employee or prospective employee has filed any complaint or instituted or caused to be instituted any proceeding under or related to this Act [29 USCS §§ 2001 et seq.],
(B) such employee or prospective employee has testified or is about to testify in any such proceeding, or
(C) of the exercise by such employee or prospective employee, on behalf of such employee or another person, of any right afforded by this Act [29 USCS §§ 2001 et seq.].
29 USCS § 2006
(e) Exemption for security services.
(1) In general. Subject to paragraph (2) and sections 8 and 10 [29 USCS §§ 2007 and 2009], this Act [29 USCS §§ 2001 et seq.] shall not prohibit the use of polygraph tests on prospective employees by any private employer whose primary business purpose consists of providing armored car personnel, personnel engaged in the design, installation, and maintenance of security alarm systems, or other uniformed or plainclothes security personnel and whose function includes protection of--
(A) facilities, materials, or operations having a significant impact on the health or safety of any State or political subdivision thereof, or the national security of the United States, as determined under rules and regulations issued by the Secretary within 90 days after the date of the enactment of this Act [June 27, 1988], including--
(i) facilities engaged in the production, transmission, or distribution of electric or nuclear power,
(ii) public water supply facilities,
(iii) shipments or storage of radioactive or other toxic waste materials, and
(iv) public transportation, or
(B) currency, negotiable securities, precious commodities or instruments, or proprietary information.
(2) Access. The exemption provided under this subsection shall not apply if the test is administered to a prospective employee who would not be employed to protect facilities, materials, operations, or assets referred to in paragraph (1).