I've found that to prevent problems it helps to keep copies of "Photographers Rights", general statutes, and court decisions regarding indecent exposure to show people when they question what you are doing in public places.
For North Carolina here are some good links that explain the following vague statute with respect to private parts...
§ 14‑190.9. Indecent exposure.
(a) Unless the conduct is punishable under subsection (a1) of this section, any person who shall willfully expose the private parts of his or her person in any public place and in the presence of any other person or persons, except for those places designated for a public purpose where the same sex exposure is incidental to a permitted activity, or aids or abets in any such act, or who procures another to perform such act; or any person, who as owner, manager, lessee, director, promoter or agent, or in any other capacity knowingly hires, leases or permits the land, building, or premises of which he is owner, lessee or tenant, or over which he has control, to be used for purposes of any such act, shall be guilty of a Class 2 misdemeanor.
(a1) Unless the conduct is prohibited by another law providing greater punishment, any person at least 18 years of age who shall willfully expose the private parts of his or her person in any public place in the presence of any other person less than 16 years of age for the purpose of arousing or gratifying sexual desire shall be guilty of a Class H felony. An offense committed under this subsection shall not be considered to be a lesser included offense under G.S. 14‑202.1.
(b) Notwithstanding any other provision of law, a woman may breast feed in any public or private location where she is otherwise authorized to be, irrespective of whether the nipple of the mother's breast is uncovered during or incidental to the breast feeding.
(c) Notwithstanding any other provision of law, a local government may regulate the location and operation of sexually oriented businesses. Such local regulation may restrict or prohibit nude, seminude, or topless dancing to the extent consistent with the constitutional protection afforded free speech. (1971, c. 591, s. 1; 1993, c. 301, s. 1; c. 539, s. 124; 1994, Ex. Sess., c. 24, s. 14(c); 1998‑46, s. 3; 2005‑226, s. 1.)
In this case the showing of buttocks was dismissed, but one judge decented with different definitions. I like the middle of the road definition that is given by the decenting judge since minors may be encountered but don't believe the strip club definition applies.
Sexually Explicit Nudity. – The showing of:
a. Uncovered, or less than opaquely covered, human genitals, pubic area, or buttocks, or the nipple or any portion of the areola of the human female breast, except as provided in G.S. 14‑190.9(b); or
b. Covered human male genitals in a discernibly turgid state. (1985, c. 703, s. 9; 1989 (Reg. Sess., 1990), c. 1022, s. 2; 1993, c. 301, s. 2; 2008‑218, s. 1.)
Please note that if you take pictures of male atheletes with a hard on even though the clothing is opaque it violates this statute. I've got some swim meet shots that may not pass this test.