This is the commonly excepted folk advice people are given, however, most state laws (US) take no account of so called posted signs. There is a ‘mens rea’ requirement that for trespassing would typically require only that the trespasser knew they were on land that did not belong to them (this issue is easier to muddy if the trespassing occurs on ‘public’ lands/property).

But the statutory basis for trespassing is most often just the act of crossing into land without prior express permission.