Land does not have to be posted or marked.
It does if you want to overcome their defense to prosecution.
39-14-405. Criminal trespass.
(a) A person commits criminal trespass if the person enters or remains on property, or any portion of property, without the consent of the owner. Consent may be inferred in the case of property that is used for commercial activity available to the general public or in the case of other property when the owner has communicated the owner's intent that the property be open to the general public.
(b) It is a defense to prosecution under this section that:
(1) A person entered or remained on property that the person reasonably believed to be property for which the owner's consent to enter had been granted;
(2) The person's conduct did not substantially interfere with the owner's use of the property; and
(3) The person immediately left the property upon request.
(c) The defenses to prosecution set out in subsection (b) shall not be applicable to a person violating this section if the property owner:
(1) Posts the property with signs that are visible at all major points of ingress to the property being posted and the signs are reasonably likely to come to the attention of a person entering the property; or
(2) Places identifying purple paint marks on trees or posts on the property; provided, that at least one (1) sign is posted at a major point of ingress to the property in a manner that is reasonably likely to come to the attention of a person entering the property and that the sign includes language describing that the use of purple paint signifies "no trespassing." If purple paint is used, then purple paint must be vertical lines of not less than eight inches (8ā³) in length and not less than one inch (1ā³) in width; placed so that the bottom of the mark is not less than three feet (3ā²) or more than five feet (5ā²) from the ground; and placed at locations that are reasonably likely to come to the attention of a person entering the property.
(d) For purposes of this section, "enter" means intrusion of the entire body or when a person causes an unmanned aircraft to enter that portion of the airspace above the owner's land not regulated as navigable airspace by the federal aviation administration.
(e) Entering or remaining on railroad or utility right-of-way property by an adjoining landowner for usual and customary activities of the type defined in Ā§Ā§ 1-3-105(a)(2)(A)(i) and (ii), (B) and (C) and 43-1-113(a), (b)(1)(A) and (B), (b)(2) and (b)(3) shall not be considered trespass under this section. This subsection (e) shall not apply if the railroad or utility right-of-way owner, by a personal communication or posting at the site by someone with either actual authority or apparent authority to act for the railroad or utility right-of-way owner, has communicated to the adjoining landowner that the activity is not permitted.
(f)
(1) The secretary of state shall establish a no trespass public notice list identifying employers in this state who have requested established private property rights to be recognized and recorded against a trespasser under subsection (a).
(2) To be included on the list, an employer shall provide to the secretary of state copies of appropriate documents that establish the employer's private property rights, including the address and legal description of the property to which it has legal control. An employer that records its private property rights shall pay a recording fee as determined by the secretary of state.
(3) Beginning January 15, 2015, and every January 15 and July 15 thereafter, the secretary of state shall:
(A) Make the list available to the public in the office of the secretary of state and publish the list on the website maintained by the secretary of state; and
(B) Distribute the no trespass public notice list to every law enforcement agency in this state.
(4) Publication of the no trespass public notice list as prescribed in subdivision (f)(3) establishes a presumption that members of the general public have notice of the establishment of private property rights of all employers and properties listed.
(5) Each law enforcement agency in this state shall maintain the most recent no trespass public notice list received from the secretary of state for its use in responding to complaints of criminal trespass under subsection (a). If a property is identified on the list, the responding law enforcement officer:
(A) Is not required to further establish an employer's property rights before taking action against a person committing criminal trespass; and
(B) May take appropriate and lawful action against a person committing criminal trespass to have such person leave the property or cease blocking ingress to or egress from the property.
(6) If the employer's property is listed on the no trespass public notice list, an owner may seek an expedited injunction to restrain repeated or continuing trespass.
(7) This subsection (f) shall not affect or limit any existing rights of an owner whose property is not included on the no trespass public notice list.
(g) Criminal trespass is a Class C misdemeanor.
(h) For purposes of this section, there shall be no inference of the owner's consent nor shall the defense in subsection (b) be available to a person entering and remaining on the grounds, or in the common areas, such as lobbies, hallways, courtyards, and parking lots, of a housing or apartment complex having signs posted in compliance with subsection (c) unless the person:
(1) Has the actual consent of the owner;
(2) May lawfully enter the property by virtue of the person's occupational duties; or
(3) Has a contractual right to enter the property or is an invitee of someone with a contractual right to make invitations to enter the property.