You can hire a licensed land surveyor to survey the property and place official markers on the boundary lines. Professional organizations, such as the California Land Surveyor's Association (californiasurveyors.org), often provide useful information on how to choose and work with a land surveyor.
The cost of a boundary survey is determined by the size of the parcel, whether it is in a subdivision, when it was last surveyed, what region of the country it is in, and other factors. A straightforward survey may cost as little as $500 in some areas of the country. But be prepared to spend $1,000 or more if no survey has been done for a long time, if the parcel is large, or if the maps are unreliable and conflicting. In fact, many surveys cost thousands of dollars.