A Survey is the only way to know for sure.
If it is blocking a retention pond then you may want to check with your HOA and see if it's owned by the community.
This is one of the reasons to always get a survey on the property when you buy it. You can eliminate these problems before they become yours.
Fill us in on what you would like to do and we might be able to help you more. Does the fence bother you and you want to remove it, does someone else "own" it, etc.
Plats are fairly generic and without knowing exactly where the property lines are, are fairly useless for this sort of situation. I would bite the bullet and hire a surveyor to mark the property lines for you so you can know for sure.