The only definitive way to determine the property line is to obtain a survey by a licensed surveyor.
Property owners are responsible for verifying their property lines. GIS can help you get an idea of where your property lines are, but these are only approximate. Use the Public Web Map, use the search bar with your Parcel ID, Name, or Property Address to find your tax parcel. This can be used as a reference to find property pins which mark your property line, though these can be moved.