First, look at the deeds to determine the points. Sometimes there are boundary markers but I'm guessing in your case there are not. There may or may not be some other markings or points referenced in the deeds to your property and that of the neighbor. If not, then the only way to determine the precise location of the boundary line is to hire a licensed surveyor.
This answer is offered for informational purposes only. It is not offered as, and does not constitute, legal advice. Laws vary widely from state to state. You should rely only on the advice given to you during a personal consultation by a local attorney who is thoroughly familiar with state laws and the area of practice in which your concern lies.
If it is platted land (e.g. a subdivision) you may be able to find the outline of your property. You can go to the County Clerk's office and see if you can find the plat. Check your original Deed to the property; it will list your legal description and tell you if it is a platted property vs. a long legal description. The easy way to tell is if it says Lot X in Subdivision Y, recorded at plat book Z, page A--at that point, you could go to the clerk and say you need a copy of the plat at book Z page A. With that being said, however, you are trying to identify a precise point on your property line. To do that, you need to find a surveyor. If it is a typical house lot (such as an acre or less) it should be in the range of $400-500. That is pretty much the only way to find your EXACT boundaries.
This is opinion, not legal advice. No attorney-client privilege has been created.