I used a rental metal detector to find the iron posts at my property corners, which after 50 years were 2" below the surface. Look by power poles which feed two adjacent houses.
BTW, the county had two surveys for my property and only one was right. The one given us with the deed was wrong.
IIRC, the books on Neighbor Law say, send your neighbors a letter explaining your position and asking them to make this right, return receipt requested.
Our neighbors to the east are jerks so we put up a fence so the east surface of the fence is just inside our property line.