I would get the land surveyed with property corner markers first.
If a neighboring property is encroaching onto yours, what are the proper steps to rectify the issue?
If the neighboring property is a new build or recently built I would contact the local Building Department and ask to review the as-built survey submitted by the Builder / Owner.
If this is not a newly created issue the first step would be to get an independent professional confirmation that there is an encroachment.
If there is, take all efforts to put on notice all involved parties including the Neighbor, Local Tax Assessor and Building/Planning Department that the encroachment exists and is not by your previous knowledge or approval. Time is not your friend. You do not want any extended period of time to elapse before acting.