Heres how it is in NY:

Q. Must I post to keep people off my property?
A. No. Trespassing is illegal even on unposted property, unless it is unimproved, apparently unused and unfenced (or not otherwise enclosed to exclude intruders.) Even on vacant land, a written notice delivered in person (or by certified mail with a signed receipt, etc.) to any person, in the name of the landowner or authorized party, containing a description of the premises and a warning of restrictions which apply has the same effect, for that person, as if the land were posted with those restrictions. Likewise, anyone asked to leave the premises, posted or not, by the landowner, occupant or other authorized person, must do so immediately.