Any notice that entry is prohibited is enough, including a fence or a posted sign. A criminal trespass warning from a store makes a return visit a new charge.
Many people assume they are not trespassing unless they have personally been told to leave first. That is not the law. Any notice that entry is prohibited is sufficient. A fence around a property is notice to stay out. A posted no trespassing sign is notice.
This also comes up with businesses open to the public. Someone accused of shoplifting is often given a criminal trespass warning barring them from that store. Returning after that warning is a new criminal trespass charge, separate from the underlying theft allegation.
Citation: Texas Penal Code §30.05
Degree and classification: Class B misdemeanor base, up to a Class A misdemeanor for a habitation, shelter center, or critical infrastructure, or a third-degree felony in narrow smuggling-related circumstances.
Statutory language: A person commits an offense if they enter or remain on property of another without effective consent, after notice that entry was forbidden or after notice to depart.