Look at the property survey in the closing documents when you purchased your home. This map shows all of the measurements that went into determining the property line, and such numbers as the distance from the house to the property line and from the house to the street should appear.
Using the measurements from the property survey, measure from the house to the identified landmarks to find the property line. If you don't have the survey any more, then contract with a local surveyor to have a new one drawn up. A surveyor's profession involves measuring and mapping property lines, and he also marks the corners of the property with a set of stakes. While a survey means spending money, having accurate knowledge of the property line helps the homeowner avoid potentially expensive litigation with neighbors.
The county recorder's office often has maps for public viewing that show property lines along different streets. Request a copy of any maps that appear to have your property lines on them, or at least the dimensions. Once you know where your property lines actually are, you know where you can install fencing and other amenities without involving your neighbor's land or engaging in some other disagreement.