Absent unusual circumstances, individuals who acquire title through adverse possession do not have an obligation to mark their property boundaries. In quiet title actions (a typical method to determine title via adverse possession), however, the lawyers/parties should have submitted a property description or map such that, when deciding the matter the Court could define the property. Accordingly, you should start with the Court Order in the matter that ended the lawsuit.
To determine the property boundary, you could hire a surveyor and then mark the boundaries (it is unlawful to disturb boundary monuments). Alternatively, the parties could agree as to a boundary line. In that case, the parties should enter into a written agreement and mark the agreed property line.