20 CFR 203.3 - When an individual is performing service for an employer.
(a) The legal relationship of employer and employee is defined by the act. Thus, an individual is performing service for an employer if:
(1) He is subject to the right of an employer, directly or through another, to supervise and direct the manner in which his services are rendered; or
(2) In rendering professional or technical services he is integrated into the staff of the employer; or
(3) He is rendering personal services on the property used in the operations of the employer and the services are integrated into those operations.
(b) The provisions in paragraph (a) of this section are controlling irrespective of whether the service is performed on a part-time basis, and, with respect to paragraph (a)(1) of this section, irrespective of whether the right to supervise and direct is exercised.